refactor: replace pagination engine with TextLayoutResult-first per-block layout

- New models: BlockMetrics (sealed), LineMetric, PageSpan, ReaderPage2,
  PaginationResult, ReaderConstraints, FingerprintKey
- New engine: PaginationEngine interface + TextMeasurerPaginationEngine
  with BlockMetricsFactory (getLineTop/Bottom/VisibleEnd) and PagePacker
  (greedy fill with widow/orphan policy + paragraph splitting)
- New infrastructure: PaginationCache (LRU), WindowedPaginationState
- UI: ReaderPaginationLayout rewritten for ReaderPage2/PageSpan rendering,
  ReaderPageContent2 renders blocks from spans with sub-sequence support
- ReaderScreen: replaced produceState with LaunchedEffect + mutableStateOf,
  uses screenModel.paginationEngine directly
- ReaderModel: injects TextMeasurerPaginationEngine, uses firstBlockIndex
  instead of startTextIndex
- Removed: PaginateReaderTextUseCase (380 lines), readerChapterTextStyle,
  TextMeasurerModule (unnecessary, TextMeasurer passed from composable)
- Removed: unused ReaderTextAlignment import from BlockMetricsFactory

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/BlockMetrics.kt

@@ -0,0 +1,42 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+import androidx.compose.ui.text.TextLayoutResult
+
+@Immutable
+sealed class BlockMetrics {
+    abstract val blockIndex: Int
+    abstract val height: Int
+
+    @Immutable
+    data class TextMetrics(
+        override val blockIndex: Int,
+        override val height: Int,
+        val lineMetrics: List<LineMetric>,
+        val layoutResult: TextLayoutResult?,
+    ) : BlockMetrics()
+
+    @Immutable
+    data class ChapterMetrics(
+        override val blockIndex: Int,
+        override val height: Int,
+    ) : BlockMetrics()
+
+    @Immutable
+    data class ImageMetrics(
+        override val blockIndex: Int,
+        override val height: Int,
+    ) : BlockMetrics()
+
+    @Immutable
+    data class SeparatorMetrics(
+        override val blockIndex: Int,
+        override val height: Int,
+    ) : BlockMetrics()
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/LineMetric.kt

@@ -0,0 +1,16 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
+data class LineMetric(
+    val top: Float,
+    val bottom: Float,
+    val visibleEnd: Int,
+)

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/PageSpan.kt

@@ -0,0 +1,26 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
+data class PageSpan(
+    val blockIndex: Int,
+    val startChar: Int,
+    val endChar: Int,
+) {
+    val isAtomic: Boolean get() = startChar == 0 && endChar == 0
+
+    companion object {
+        fun atomic(blockIndex: Int) = PageSpan(
+            blockIndex = blockIndex,
+            startChar = 0,
+            endChar = 0,
+        )
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/PaginationResult.kt

@@ -0,0 +1,22 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
+data class PaginationResult(
+    val pages: List<ReaderPage2>,
+    val totalBlocks: Int,
+)
+
+@Immutable
+data class FingerprintKey(
+    val contentHash: Int,
+    val styleFingerprint: Int,
+    val constraintsFingerprint: Int,
+)

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/ReaderConstraints.kt

@@ -0,0 +1,29 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
+data class ReaderConstraints(
+    val contentWidthPx: Int,
+    val contentHeightPx: Int,
+    val paragraphSpacingPx: Int,
+    val safetyMarginPx: Int = 4,
+    val minWidowLines: Int = 2,
+    val minOrphanLines: Int = 2,
+    val chapterKeepLines: Int = 2,
+    val separatorHeightPx: Int = DEFAULT_SEPARATOR_HEIGHT_PX,
+    val defaultImageHeightPx: Int = DEFAULT_IMAGE_HEIGHT_PX,
+) {
+    val availableHeight: Int get() = (contentHeightPx - safetyMarginPx).coerceAtLeast(1)
+
+    companion object {
+        private const val DEFAULT_SEPARATOR_HEIGHT_PX = 12
+        private const val DEFAULT_IMAGE_HEIGHT_PX = 720
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/model/reader/pagination/ReaderPage2.kt

@@ -0,0 +1,18 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.model.reader.pagination
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
+data class ReaderPage2(
+    val index: Int,
+    val spans: List<PageSpan>,
+) {
+    val firstBlockIndex: Int get() = spans.firstOrNull()?.blockIndex ?: 0
+    val lastBlockIndex: Int get() = spans.lastOrNull()?.blockIndex ?: 0
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/PaginateReaderTextUseCase.kt

@@ -1,380 +0,0 @@
-/*
- * Book's Story — free and open-source Material You eBook reader.
- * Copyright (C) 2024-2026 Acclorite
- * SPDX-License-Identifier: GPL-3.0-only
- */
-
-package org.dueattendant149.bookshelf.domain.use_case.reader
-
-import android.util.Log
-import androidx.compose.ui.text.AnnotatedString
-import androidx.compose.ui.text.TextLayoutResult
-import androidx.compose.ui.text.TextMeasurer
-import androidx.compose.ui.text.TextStyle
-import androidx.compose.ui.unit.Constraints
-import org.dueattendant149.bookshelf.domain.model.reader.ReaderPage
-import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
-import org.dueattendant149.bookshelf.presentation.reader.model.ReaderTextAlignment
-import org.dueattendant149.bookshelf.ui.reader.readerChapterTextStyle
-import javax.inject.Inject
-import javax.inject.Singleton
-
-@Singleton
-class PaginateReaderTextUseCase @Inject constructor() {
-
-    operator fun invoke(
-        text: List<ReaderText>,
-        contentWidthPx: Int,
-        contentHeightPx: Int,
-        paragraphStyle: TextStyle,
-        chapterTitleAlignment: ReaderTextAlignment,
-        paragraphSpacingPx: Int,
-        textMeasurer: TextMeasurer,
-        imageMaxWidthPx: Int,
-        chapterKeepLines: Int = 2,
-        minWidowLines: Int = 2,
-        minOrphanLines: Int = 2,
-        safetyHeightPx: Int = 4,
-        chapterExtraHeightPx: Int = 0,
-        separatorHeightPx: Int = DEFAULT_SEPARATOR_HEIGHT_PX,
-        defaultImageHeightPx: Int = DEFAULT_IMAGE_HEIGHT_PX,
-    ): List<ReaderPage> {
-        if (text.isEmpty() || contentWidthPx <= 0 || contentHeightPx <= 0) return emptyList()
-
-        val availableHeight = (contentHeightPx - safetyHeightPx).coerceAtLeast(1)
-        val workingBlocks = text.toMutableList()
-        val metrics =         workingBlocks.map { block ->
-            measureBlock(
-                block = block,
-                contentWidthPx = contentWidthPx,
-                imageMaxWidthPx = imageMaxWidthPx,
-                paragraphStyle = paragraphStyle,
-                chapterTitleAlignment = chapterTitleAlignment,
-                textMeasurer = textMeasurer,
-                chapterExtraHeightPx = chapterExtraHeightPx,
-                separatorHeightPx = separatorHeightPx,
-                defaultImageHeightPx = defaultImageHeightPx,
-            )
-        }.toMutableList()
-
-        val pages = mutableListOf<ReaderPage>()
-        val pageItems = mutableListOf<ReaderText>()
-        var startTextIndex = 0
-        var remainingHeight = availableHeight
-        var currentPageIndex = 0
-        var index = 0
-
-        /**
-         * Returns the height of [metric] plus the inter-item spacing if this
-         * block is not the first one on the current page. This matches the
-         * [Arrangement.spacedBy] used in the paginated reader UI.
-         */
-        fun heightWithSpacing(metric: BlockMetrics): Int {
-            return if (pageItems.isEmpty()) metric.height else metric.height + paragraphSpacingPx
-        }
-
-        fun emitPage(endTextIndex: Int) {
-            if (pageItems.isEmpty()) return
-            pages.add(
-                ReaderPage(
-                    index = currentPageIndex,
-                    items = pageItems.toList(),
-                    startTextIndex = startTextIndex,
-                    endTextIndex = endTextIndex,
-                )
-            )
-            currentPageIndex++
-            pageItems.clear()
-            remainingHeight = availableHeight
-            startTextIndex = endTextIndex
-        }
-
-        while (index < workingBlocks.size) {
-            val block = workingBlocks[index]
-            val metric = metrics[index]
-            val blockHeight = heightWithSpacing(metric)
-
-            when (block) {
-                is ReaderText.Chapter -> {
-                    val keep = followingTextKeepHeight(
-                        blocks = workingBlocks,
-                        metrics = metrics,
-                        startIndex = index + 1,
-                        keepLines = chapterKeepLines,
-                    )
-                    // Heading itself + spacing after it (if anything follows) + kept text.
-                    val requiredWithKeep = blockHeight +
-                            (if (index < workingBlocks.lastIndex) paragraphSpacingPx else 0) +
-                            keep
-
-                    when {
-                        requiredWithKeep > availableHeight -> {
-                            // Heading + kept lines don't fit on a fresh page.
-                            // Place heading alone.
-                            if (blockHeight > remainingHeight && pageItems.isNotEmpty()) {
-                                emitPage(index)
-                            }
-                            pageItems.add(block)
-                            remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
-                            index++
-                        }
-
-                        requiredWithKeep > remainingHeight && pageItems.isNotEmpty() -> {
-                            // Move heading to the next page so it stays with following text.
-                            emitPage(index)
-                        }
-
-                        else -> {
-                            pageItems.add(block)
-                            remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
-                            index++
-                        }
-                    }
-                }
-
-                is ReaderText.Separator,
-                is ReaderText.Image -> {
-                    if (blockHeight > remainingHeight && pageItems.isNotEmpty()) {
-                        emitPage(index)
-                    }
-                    pageItems.add(block)
-                    remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
-                    index++
-                }
-
-                is ReaderText.Text -> {
-                    if (blockHeight <= remainingHeight) {
-                        pageItems.add(block)
-                        remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
-                        index++
-                    } else if (pageItems.isNotEmpty()) {
-                        emitPage(index)
-                        // Retry the same block on a fresh page.
-                    } else {
-                        // Fresh page and the paragraph still doesn't fit whole.
-                        val split = splitParagraph(
-                            block = block,
-                            metric = metric,
-                            availableHeight = remainingHeight,
-                            minWidowLines = minWidowLines,
-                            minOrphanLines = minOrphanLines,
-                        )
-
-                        if (split == null) {
-                            // Whole paragraph fits on a fresh page but we shouldn't be here.
-                            // Safety fallback: place it anyway.
-                            pageItems.add(block)
-                            remainingHeight = 0
-                            index++
-                        } else {
-                            val (head, tail) = split
-                            pageItems.add(ReaderText.Text(head))
-                            emitPage(index + 1)
-                            // The remainder is still the same logical block, so the next page
-                            // should start at this index rather than index + 1.
-                            startTextIndex = index
-
-                            if (tail.isBlank()) {
-                                index++
-                            } else {
-                                // Replace current block with remainder and retry on fresh page.
-                                val remainderBlock = ReaderText.Text(tail)
-                                workingBlocks[index] = remainderBlock
-                                metrics[index] = measureBlock(
-                                    block = remainderBlock,
-                                    contentWidthPx = contentWidthPx,
-                                    imageMaxWidthPx = imageMaxWidthPx,
-                                    paragraphStyle = paragraphStyle,
-                                    chapterTitleAlignment = chapterTitleAlignment,
-                                    textMeasurer = textMeasurer,
-                                    chapterExtraHeightPx = chapterExtraHeightPx,
-                                    separatorHeightPx = separatorHeightPx,
-                                    defaultImageHeightPx = defaultImageHeightPx,
-                                )
-                            }
-                        }
-                    }
-                }
-            }
-        }
-
-        if (pageItems.isNotEmpty()) {
-            emitPage(workingBlocks.size)
-        }
-
-        Log.d("Pagination", "Split ${text.size} blocks into ${pages.size} pages")
-        return pages
-    }
-
-    private fun measureBlock(
-        block: ReaderText,
-        contentWidthPx: Int,
-        imageMaxWidthPx: Int,
-        paragraphStyle: TextStyle,
-        chapterTitleAlignment: ReaderTextAlignment,
-        textMeasurer: TextMeasurer,
-        chapterExtraHeightPx: Int,
-        separatorHeightPx: Int,
-        defaultImageHeightPx: Int,
-    ): BlockMetrics {
-        return when (block) {
-            is ReaderText.Text -> {
-                val layout = measureText(block.line, paragraphStyle, contentWidthPx, textMeasurer)
-                val textHeight = layout?.size?.height ?: 0
-                val lineCount = layout?.lineCount ?: 0
-                val lineHeights = (0 until lineCount).map { lineIndex ->
-                    layout!!.getLineBottom(lineIndex)
-                }
-                BlockMetrics(
-                    height = textHeight,
-                    lineHeights = lineHeights,
-                    isSplittable = true,
-                    layout = layout,
-                )
-            }
-
-            is ReaderText.Chapter -> {
-                val chapterStyle = readerChapterTextStyle(
-                    nested = block.nested,
-                    textAlignment = chapterTitleAlignment,
-                )
-                val layout = measureText(
-                    AnnotatedString(block.title),
-                    chapterStyle,
-                    contentWidthPx,
-                    textMeasurer,
-                )
-                val titleHeight = layout?.size?.height ?: 0
-                BlockMetrics(
-                    height = titleHeight + chapterExtraHeightPx,
-                    isSplittable = false,
-                )
-            }
-
-            is ReaderText.Separator -> BlockMetrics(
-                height = separatorHeightPx,
-                isSplittable = false,
-            )
-
-            is ReaderText.Image -> BlockMetrics(
-                height = imageHeight(block, imageMaxWidthPx, defaultImageHeightPx),
-                isSplittable = false,
-            )
-        }
-    }
-
-    /**
-     * Returns the height of the first [keepLines] lines of the text block at
-     * [startIndex], or the full height of the block if it has fewer lines. If
-     * the next block is not splittable (e.g. an image), its full height is
-     * returned so the heading can be moved to the next page when necessary.
-     */
-    private fun followingTextKeepHeight(
-        blocks: List<ReaderText>,
-        metrics: List<BlockMetrics>,
-        startIndex: Int,
-        keepLines: Int,
-    ): Int {
-        if (startIndex >= blocks.size) return 0
-        val metric = metrics[startIndex]
-        val block = blocks[startIndex]
-
-        if (metric.lineHeights.isEmpty()) return metric.height
-
-        if (block !is ReaderText.Text) {
-            // Keep the whole non-text block with the heading.
-            return metric.height
-        }
-
-        val linesToKeep = minOf(keepLines, metric.lineHeights.size)
-        return metric.lineHeights[linesToKeep - 1].toInt()
-    }
-
-    private fun splitParagraph(
-        block: ReaderText.Text,
-        metric: BlockMetrics,
-        availableHeight: Int,
-        minWidowLines: Int,
-        minOrphanLines: Int,
-    ): Pair<AnnotatedString, AnnotatedString>? {
-        val layout = metric.layout ?: return null
-        val lineCount = layout.lineCount
-        if (lineCount == 0) return null
-
-        // Find the last line whose bottom fits inside the available height.
-        val lastFittingLine = layout.getLineForVerticalPosition(availableHeight.toFloat())
-            .coerceIn(0, lineCount - 1)
-
-        val maxLinesCurrent = lastFittingLine + 1
-        val minLinesCurrent = minWidowLines.coerceAtLeast(1)
-        val minLinesRemainder = minOrphanLines.coerceAtLeast(1)
-
-        // Ideal break: current page has at least minWidowLines,
-        // remainder has at least minOrphanLines.
-        val idealMax = (lineCount - minLinesRemainder).coerceAtLeast(minLinesCurrent)
-        val candidateLines = maxLinesCurrent.coerceIn(minLinesCurrent, idealMax)
-
-        if (candidateLines in minLinesCurrent..idealMax) {
-            val breakChar = layout.getLineEnd(candidateLines - 1, visibleEnd = true)
-            if (breakChar in 1 until block.line.length) {
-                return block.line.subSequence(0, breakChar) to
-                    block.line.subSequence(breakChar, block.line.length)
-            }
-        }
-
-        // Paragraph is longer than a full page or constraints cannot be satisfied.
-        // Split at the last line that actually fits.
-        val forcedLines = maxLinesCurrent.coerceIn(1, lineCount - 1)
-        val breakChar = layout.getLineEnd(forcedLines - 1, visibleEnd = true)
-        if (breakChar in 1 until block.line.length) {
-            return block.line.subSequence(0, breakChar) to
-                block.line.subSequence(breakChar, block.line.length)
-        }
-
-        return null
-    }
-
-    private fun measureText(
-        text: AnnotatedString,
-        style: TextStyle,
-        maxWidthPx: Int,
-        textMeasurer: TextMeasurer,
-    ): TextLayoutResult? {
-        if (text.isBlank()) return null
-        return try {
-            textMeasurer.measure(
-                text = text,
-                style = style,
-                constraints = Constraints(maxWidth = maxWidthPx),
-                softWrap = true,
-            )
-        } catch (e: Exception) {
-            Log.e("Pagination", "measure failed", e)
-            null
-        }
-    }
-
-    private fun imageHeight(
-        block: ReaderText.Image,
-        imageMaxWidthPx: Int,
-        defaultImageHeightPx: Int,
-    ): Int {
-        val bitmap = block.imageBitmap
-        val width = bitmap.width
-        val height = bitmap.height
-        if (width <= 0 || height <= 0) return defaultImageHeightPx
-        return (imageMaxWidthPx * height / width).toInt()
-    }
-
-    private data class BlockMetrics(
-        val height: Int,
-        val lineHeights: List<Float> = emptyList(),
-        val isSplittable: Boolean = false,
-        val layout: TextLayoutResult? = null,
-    )
-
-    companion object {
-        private const val DEFAULT_SEPARATOR_HEIGHT_PX = 12
-        private const val DEFAULT_IMAGE_HEIGHT_PX = 720
-    }
-}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/BlockMetricsFactory.kt

@@ -0,0 +1,150 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import android.util.Log
+import androidx.compose.ui.text.AnnotatedString
+import androidx.compose.ui.text.PlatformTextStyle
+import androidx.compose.ui.text.TextLayoutResult
+import androidx.compose.ui.text.TextMeasurer
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.text.style.TextIndent
+import androidx.compose.ui.unit.Constraints
+import androidx.compose.ui.unit.sp
+import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.BlockMetrics
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.LineMetric
+
+object BlockMetricsFactory {
+
+    fun measureAll(
+        blocks: List<ReaderText>,
+        contentWidthPx: Int,
+        imageMaxWidthPx: Int,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        chapterExtraHeightPx: Int,
+        separatorHeightPx: Int,
+        defaultImageHeightPx: Int,
+        chapterTitleAlignment: TextAlign = TextAlign.Start,
+    ): List<BlockMetrics> {
+        return blocks.mapIndexed { index, block ->
+            measureBlock(
+                block = block,
+                blockIndex = index,
+                contentWidthPx = contentWidthPx,
+                imageMaxWidthPx = imageMaxWidthPx,
+                paragraphStyle = paragraphStyle,
+                textMeasurer = textMeasurer,
+                chapterExtraHeightPx = chapterExtraHeightPx,
+                separatorHeightPx = separatorHeightPx,
+                defaultImageHeightPx = defaultImageHeightPx,
+                chapterTitleAlignment = chapterTitleAlignment,
+            )
+        }
+    }
+
+    private fun measureBlock(
+        block: ReaderText,
+        blockIndex: Int,
+        contentWidthPx: Int,
+        imageMaxWidthPx: Int,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        chapterExtraHeightPx: Int,
+        separatorHeightPx: Int,
+        defaultImageHeightPx: Int,
+        chapterTitleAlignment: TextAlign = TextAlign.Start,
+    ): BlockMetrics {
+        return when (block) {
+            is ReaderText.Text -> {
+                val layout = measureText(block.line, paragraphStyle, contentWidthPx, textMeasurer)
+                val lineMetrics = if (layout != null) {
+                    (0 until layout.lineCount).map { lineIndex ->
+                        LineMetric(
+                            top = layout.getLineTop(lineIndex),
+                            bottom = layout.getLineBottom(lineIndex),
+                            visibleEnd = layout.getLineEnd(lineIndex, visibleEnd = true),
+                        )
+                    }
+                } else emptyList()
+                val height = layout?.size?.height ?: 0
+                BlockMetrics.TextMetrics(
+                    blockIndex = blockIndex,
+                    height = height,
+                    lineMetrics = lineMetrics,
+                    layoutResult = layout,
+                )
+            }
+
+            is ReaderText.Chapter -> {
+                val chapterStyle = TextStyle(
+                    fontSize = if (block.nested) 24.sp else 28.sp,
+                    lineHeight = if (block.nested) 32.sp else 36.sp,
+                    fontWeight = FontWeight.Normal,
+                    textAlign = chapterTitleAlignment,
+                    platformStyle = PlatformTextStyle(includeFontPadding = false),
+                )
+                val layout = measureText(
+                    text = AnnotatedString(block.title),
+                    style = chapterStyle,
+                    maxWidthPx = contentWidthPx,
+                    textMeasurer = textMeasurer,
+                )
+                val titleHeight = layout?.size?.height ?: 0
+                BlockMetrics.ChapterMetrics(
+                    blockIndex = blockIndex,
+                    height = titleHeight + chapterExtraHeightPx,
+                )
+            }
+
+            is ReaderText.Separator -> BlockMetrics.SeparatorMetrics(
+                blockIndex = blockIndex,
+                height = separatorHeightPx,
+            )
+
+            is ReaderText.Image -> BlockMetrics.ImageMetrics(
+                blockIndex = blockIndex,
+                height = imageHeight(block, imageMaxWidthPx, defaultImageHeightPx),
+            )
+        }
+    }
+
+    private fun measureText(
+        text: AnnotatedString,
+        style: TextStyle,
+        maxWidthPx: Int,
+        textMeasurer: TextMeasurer,
+    ): TextLayoutResult? {
+        if (text.isBlank()) return null
+        return try {
+            textMeasurer.measure(
+                text = text,
+                style = style,
+                constraints = Constraints(maxWidth = maxWidthPx),
+                softWrap = true,
+            )
+        } catch (e: Exception) {
+            Log.e("BlockMetricsFactory", "measure failed", e)
+            null
+        }
+    }
+
+    private fun imageHeight(
+        block: ReaderText.Image,
+        imageMaxWidthPx: Int,
+        defaultImageHeightPx: Int,
+    ): Int {
+        val bitmap = block.imageBitmap
+        val width = bitmap.width
+        val height = bitmap.height
+        if (width <= 0 || height <= 0) return defaultImageHeightPx
+        return (imageMaxWidthPx * height / width).toInt()
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/PagePacker.kt

@@ -0,0 +1,189 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import android.util.Log
+import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.BlockMetrics
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PageSpan
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PaginationResult
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderConstraints
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
+
+object PagePacker {
+
+    fun pack(
+        blocks: List<ReaderText>,
+        metrics: List<BlockMetrics>,
+        constraints: ReaderConstraints,
+        reMeasureText: (ReaderText.Text, Int) -> BlockMetrics,
+    ): PaginationResult {
+        if (blocks.isEmpty() || metrics.isEmpty()) {
+            return PaginationResult(pages = emptyList(), totalBlocks = blocks.size)
+        }
+
+        val workingBlocks = blocks.toMutableList()
+        val workingMetrics = metrics.toMutableList()
+        val availableHeight = constraints.availableHeight
+        val pages = mutableListOf<ReaderPage2>()
+        val currentSpans = mutableListOf<PageSpan>()
+        var remainingHeight = availableHeight
+        var pageIndex = 0
+
+        fun heightWithSpacing(metric: BlockMetrics): Int {
+            return if (currentSpans.isEmpty()) metric.height
+            else metric.height + constraints.paragraphSpacingPx
+        }
+
+        fun emitPage() {
+            if (currentSpans.isEmpty()) return
+            pages.add(ReaderPage2(index = pageIndex, spans = currentSpans.toList()))
+            pageIndex++
+            currentSpans.clear()
+            remainingHeight = availableHeight
+        }
+
+        var i = 0
+        while (i < workingBlocks.size) {
+            val block = workingBlocks[i]
+            val metric = workingMetrics[i]
+            val blockHeight = heightWithSpacing(metric)
+
+            when (metric) {
+                is BlockMetrics.TextMetrics -> {
+                    if (blockHeight <= remainingHeight) {
+                        currentSpans.add(PageSpan.atomic(i))
+                        remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
+                        i++
+                    } else if (currentSpans.isNotEmpty()) {
+                        emitPage()
+                    } else {
+                        val split = splitParagraph(
+                            block = block as ReaderText.Text,
+                            metric = metric,
+                            availableHeight = remainingHeight,
+                            minWidowLines = constraints.minWidowLines,
+                            minOrphanLines = constraints.minOrphanLines,
+                        )
+                        if (split == null) {
+                            currentSpans.add(PageSpan.atomic(i))
+                            remainingHeight = 0
+                            i++
+                        } else {
+                            val (headEndChar, tailText) = split
+                            currentSpans.add(PageSpan(blockIndex = i, startChar = 0, endChar = headEndChar))
+                            emitPage()
+
+                            if (tailText.isBlank()) {
+                                i++
+                            } else {
+                                val tailBlock = ReaderText.Text(tailText)
+                                workingBlocks[i] = tailBlock
+                                workingMetrics[i] = reMeasureText(tailBlock, i)
+                            }
+                        }
+                    }
+                }
+
+                is BlockMetrics.ChapterMetrics -> {
+                    val keep = followingTextKeepHeight(
+                        metrics = workingMetrics,
+                        startIndex = i + 1,
+                        keepLines = constraints.chapterKeepLines,
+                    )
+                    val chapterSpacing = if (i < workingBlocks.lastIndex) constraints.paragraphSpacingPx else 0
+                    val requiredWithKeep = blockHeight + chapterSpacing + keep
+
+                    when {
+                        requiredWithKeep > availableHeight -> {
+                            if (blockHeight > remainingHeight && currentSpans.isNotEmpty()) {
+                                emitPage()
+                            }
+                            currentSpans.add(PageSpan.atomic(i))
+                            remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
+                            i++
+                        }
+                        requiredWithKeep > remainingHeight && currentSpans.isNotEmpty() -> {
+                            emitPage()
+                        }
+                        else -> {
+                            currentSpans.add(PageSpan.atomic(i))
+                            remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
+                            i++
+                        }
+                    }
+                }
+
+                is BlockMetrics.ImageMetrics,
+                is BlockMetrics.SeparatorMetrics -> {
+                    if (blockHeight > remainingHeight && currentSpans.isNotEmpty()) {
+                        emitPage()
+                    }
+                    currentSpans.add(PageSpan.atomic(i))
+                    remainingHeight = (remainingHeight - blockHeight).coerceAtLeast(0)
+                    i++
+                }
+            }
+        }
+
+        if (currentSpans.isNotEmpty()) {
+            emitPage()
+        }
+
+        Log.d("PagePacker", "Packed ${blocks.size} blocks into ${pages.size} pages")
+        return PaginationResult(pages = pages, totalBlocks = blocks.size)
+    }
+
+    private fun splitParagraph(
+        block: ReaderText.Text,
+        metric: BlockMetrics.TextMetrics,
+        availableHeight: Int,
+        minWidowLines: Int,
+        minOrphanLines: Int,
+    ): Pair<Int, androidx.compose.ui.text.AnnotatedString>? {
+        val layout = metric.layoutResult ?: return null
+        val lineCount = layout.lineCount
+        if (lineCount == 0) return null
+
+        val lastFittingLine = layout.getLineForVerticalPosition(availableHeight.toFloat())
+            .coerceIn(0, lineCount - 1)
+
+        val maxLinesCurrent = lastFittingLine + 1
+        val minLinesCurrent = minWidowLines.coerceAtLeast(1)
+        val minLinesRemainder = minOrphanLines.coerceAtLeast(1)
+        val idealMax = (lineCount - minLinesRemainder).coerceAtLeast(minLinesCurrent)
+        val candidateLines = maxLinesCurrent.coerceIn(minLinesCurrent, idealMax)
+
+        if (candidateLines in minLinesCurrent..idealMax) {
+            val breakChar = layout.getLineEnd(candidateLines - 1, visibleEnd = true)
+            if (breakChar in 1 until block.line.length) {
+                return breakChar to block.line.subSequence(breakChar, block.line.length)
+            }
+        }
+
+        val forcedLines = maxLinesCurrent.coerceIn(1, lineCount - 1)
+        val breakChar = layout.getLineEnd(forcedLines - 1, visibleEnd = true)
+        if (breakChar in 1 until block.line.length) {
+            return breakChar to block.line.subSequence(breakChar, block.line.length)
+        }
+
+        return null
+    }
+
+    private fun followingTextKeepHeight(
+        metrics: List<BlockMetrics>,
+        startIndex: Int,
+        keepLines: Int,
+    ): Int {
+        if (startIndex >= metrics.size) return 0
+        val metric = metrics[startIndex]
+        if (metric !is BlockMetrics.TextMetrics) return metric.height
+        val linesToKeep = minOf(keepLines, metric.lineMetrics.size)
+        if (linesToKeep == 0) return 0
+        return metric.lineMetrics[linesToKeep - 1].bottom.toInt()
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/PaginationCache.kt

@@ -0,0 +1,36 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.FingerprintKey
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PaginationResult
+import javax.inject.Inject
+import javax.inject.Singleton
+
+@Singleton
+class PaginationCache @Inject constructor(
+    private val maxSize: Int = 16,
+) {
+    private val cache = object : LinkedHashMap<FingerprintKey, PaginationResult>() {
+        override fun removeEldestEntry(eldest: Map.Entry<FingerprintKey, PaginationResult>): Boolean {
+            return size > maxSize
+        }
+    }
+
+    @Synchronized
+    fun get(key: FingerprintKey): PaginationResult? = cache[key]
+
+    @Synchronized
+    fun put(key: FingerprintKey, result: PaginationResult) {
+        cache[key] = result
+    }
+
+    @Synchronized
+    fun clear() {
+        cache.clear()
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/PaginationEngine.kt

@@ -0,0 +1,26 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import androidx.compose.ui.text.TextMeasurer
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.style.TextAlign
+import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PaginationResult
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderConstraints
+
+interface PaginationEngine {
+    fun paginate(
+        blocks: List<ReaderText>,
+        constraints: ReaderConstraints,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        imageMaxWidthPx: Int,
+        chapterExtraHeightPx: Int,
+        chapterTitleAlignment: TextAlign = TextAlign.Start,
+    ): PaginationResult
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/TextMeasurerPaginationEngine.kt

@@ -0,0 +1,90 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import androidx.compose.ui.text.TextMeasurer
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.style.TextAlign
+import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.BlockMetrics
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PaginationResult
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderConstraints
+import javax.inject.Inject
+import javax.inject.Singleton
+
+@Singleton
+class TextMeasurerPaginationEngine @Inject constructor() : PaginationEngine {
+
+    override fun paginate(
+        blocks: List<ReaderText>,
+        constraints: ReaderConstraints,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        imageMaxWidthPx: Int,
+        chapterExtraHeightPx: Int,
+        chapterTitleAlignment: TextAlign,
+    ): PaginationResult {
+        val metrics = BlockMetricsFactory.measureAll(
+            blocks = blocks,
+            contentWidthPx = constraints.contentWidthPx,
+            imageMaxWidthPx = imageMaxWidthPx,
+            paragraphStyle = paragraphStyle,
+            textMeasurer = textMeasurer,
+            chapterExtraHeightPx = chapterExtraHeightPx,
+            separatorHeightPx = constraints.separatorHeightPx,
+            defaultImageHeightPx = constraints.defaultImageHeightPx,
+            chapterTitleAlignment = chapterTitleAlignment,
+        )
+
+        return PagePacker.pack(
+            blocks = blocks,
+            metrics = metrics,
+            constraints = constraints,
+            reMeasureText = { tailBlock, blockIndex ->
+                measureSingleBlock(
+                    block = tailBlock,
+                    blockIndex = blockIndex,
+                    contentWidthPx = constraints.contentWidthPx,
+                    imageMaxWidthPx = imageMaxWidthPx,
+                    paragraphStyle = paragraphStyle,
+                    textMeasurer = textMeasurer,
+                    chapterExtraHeightPx = chapterExtraHeightPx,
+                    separatorHeightPx = constraints.separatorHeightPx,
+                    defaultImageHeightPx = constraints.defaultImageHeightPx,
+                )
+            },
+        )
+    }
+
+    private fun measureSingleBlock(
+        block: ReaderText.Text,
+        blockIndex: Int,
+        contentWidthPx: Int,
+        imageMaxWidthPx: Int,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        chapterExtraHeightPx: Int,
+        separatorHeightPx: Int,
+        defaultImageHeightPx: Int,
+    ): BlockMetrics {
+        val measured = BlockMetricsFactory.measureAll(
+            blocks = listOf(block),
+            contentWidthPx = contentWidthPx,
+            imageMaxWidthPx = imageMaxWidthPx,
+            paragraphStyle = paragraphStyle,
+            textMeasurer = textMeasurer,
+            chapterExtraHeightPx = chapterExtraHeightPx,
+            separatorHeightPx = separatorHeightPx,
+            defaultImageHeightPx = defaultImageHeightPx,
+        ).first()
+        return if (measured is BlockMetrics.TextMetrics) {
+            measured.copy(blockIndex = blockIndex)
+        } else {
+            measured
+        }
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/domain/use_case/reader/pagination/WindowedPaginationState.kt

@@ -0,0 +1,122 @@
+/*
+ * Book's Story — free and open-source Material You eBook reader.
+ * Copyright (C) 2024-2026 Acclorite
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package org.dueattendant149.bookshelf.domain.use_case.reader.pagination
+
+import androidx.compose.ui.text.TextMeasurer
+import androidx.compose.ui.text.TextStyle
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.withContext
+import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.FingerprintKey
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.PaginationResult
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderConstraints
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
+import javax.inject.Inject
+import javax.inject.Singleton
+import kotlin.math.abs
+
+@Singleton
+class WindowedPaginationState @Inject constructor(
+    private val engine: TextMeasurerPaginationEngine,
+    private val cache: PaginationCache,
+) {
+    private val _result = MutableStateFlow(PaginationResult(emptyList(), 0))
+    val result: StateFlow<PaginationResult> = _result.asStateFlow()
+
+    private val _currentPage = MutableStateFlow(0)
+    val currentPage: StateFlow<Int> = _currentPage.asStateFlow()
+
+    private val _windowSize = MutableStateFlow(WINDOW_SIZE)
+    val windowSize: StateFlow<Int> = _windowSize.asStateFlow()
+
+    private var lastKey: FingerprintKey? = null
+
+    suspend fun refresh(
+        blocks: List<ReaderText>,
+        constraints: ReaderConstraints,
+        paragraphStyle: TextStyle,
+        textMeasurer: TextMeasurer,
+        imageMaxWidthPx: Int,
+        chapterExtraHeightPx: Int,
+        dispatcher: CoroutineDispatcher = Dispatchers.Default,
+    ) {
+        if (blocks.isEmpty()) {
+            _result.value = PaginationResult(emptyList(), 0)
+            _currentPage.value = 0
+            return
+        }
+
+        val key = FingerprintKey(
+            contentHash = blocks.hashCode(),
+            styleFingerprint = paragraphStyle.hashCode(),
+            constraintsFingerprint = constraints.hashCode(),
+        )
+
+        if (key == lastKey) return
+
+        val cached = cache.get(key)
+        if (cached != null) {
+            _result.value = cached
+            lastKey = key
+            return
+        }
+
+        val computed = withContext(dispatcher) {
+            engine.paginate(
+                blocks = blocks,
+                constraints = constraints,
+                paragraphStyle = paragraphStyle,
+                textMeasurer = textMeasurer,
+                imageMaxWidthPx = imageMaxWidthPx,
+                chapterExtraHeightPx = chapterExtraHeightPx,
+            )
+        }
+
+        cache.put(key, computed)
+        _result.value = computed
+        lastKey = key
+    }
+
+    fun selectPage(index: Int) {
+        val pages = _result.value.pages
+        if (pages.isEmpty()) {
+            _currentPage.value = 0
+            return
+        }
+        _currentPage.value = index.coerceIn(0, pages.lastIndex)
+    }
+
+    fun restorePage(savedBlockIndex: Int): Int {
+        val pages = _result.value.pages
+        if (pages.isEmpty()) return 0
+        val found = pages.indexOfFirst { it.firstBlockIndex <= savedBlockIndex && savedBlockIndex <= it.lastBlockIndex }
+        return found.takeIf { it >= 0 } ?: 0
+    }
+
+    fun visiblePages(): List<ReaderPage2> {
+        val pages = _result.value.pages
+        val current = _currentPage.value
+        val window = _windowSize.value
+        val from = (current - window).coerceAtLeast(0)
+        val to = (current + window + 1).coerceAtMost(pages.size)
+        return pages.subList(from, to)
+    }
+
+    fun clear() {
+        _result.value = PaginationResult(emptyList(), 0)
+        _currentPage.value = 0
+        lastKey = null
+    }
+
+    companion object {
+        private const val WINDOW_SIZE = 3
+    }
+}

app/src/main/java/org/dueattendant149/bookshelf/presentation/reader/ReaderEvent.kt

@@ -8,6 +8,7 @@ package org.dueattendant149.bookshelf.presentation.reader
 
 import androidx.compose.runtime.Immutable
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText.Chapter
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.presentation.reader.model.Checkpoint
 
 @Immutable
@@ -45,7 +46,7 @@ sealed class ReaderEvent {
     ) : ReaderEvent()
 
     data class OnPagesComputed(
-        val pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>
+        val pages: List<ReaderPage2>
     ) : ReaderEvent()
 
     data class OnRestoreCheckpoint(
@@ -90,4 +91,4 @@ sealed class ReaderEvent {
     data class OnNavigateToRsvp(
         val bookId: Int
     ) : ReaderEvent()
-}
+}

app/src/main/java/org/dueattendant149/bookshelf/presentation/reader/ReaderModel.kt

@@ -32,12 +32,13 @@ import org.dueattendant149.bookshelf.R
 import org.dueattendant149.bookshelf.core.helpers.coerceAndPreventNaN
 import org.dueattendant149.bookshelf.core.ui.UIText
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText.Chapter
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.domain.use_case.book.GetBookUseCase
 import org.dueattendant149.bookshelf.domain.use_case.book.GetChapterProgressUseCase
 import org.dueattendant149.bookshelf.domain.use_case.book.GetTextUseCase
 import org.dueattendant149.bookshelf.domain.use_case.book.UpdateBookUseCase
 import org.dueattendant149.bookshelf.domain.use_case.history.GetHistoryForBookUseCase
-import org.dueattendant149.bookshelf.domain.use_case.reader.PaginateReaderTextUseCase
+import org.dueattendant149.bookshelf.domain.use_case.reader.pagination.TextMeasurerPaginationEngine
 import org.dueattendant149.bookshelf.domain.use_case.remote.SyncReadingProgressUseCase
 import org.dueattendant149.bookshelf.presentation.history.HistoryScreen
 import org.dueattendant149.bookshelf.presentation.library.LibraryScreen
@@ -54,7 +55,7 @@ class ReaderModel @Inject constructor(
     private val getHistoryForBookUseCase: GetHistoryForBookUseCase,
     private val getChapterProgressUseCase: GetChapterProgressUseCase,
     private val syncReadingProgressUseCase: SyncReadingProgressUseCase,
-    private val paginateReaderTextUseCase: PaginateReaderTextUseCase,
+    val paginationEngine: TextMeasurerPaginationEngine,
 ) : ViewModel() {
 
     private val mutex = Mutex()
@@ -118,7 +119,7 @@ class ReaderModel @Inject constructor(
                     val state = _state.value
                     if (state.pages.isNotEmpty()) {
                         val page = state.pages
-                            .indexOfFirst { it.startTextIndex <= state.book.scrollIndex && state.book.scrollIndex < it.endTextIndex }
+                            .indexOfFirst { it.firstBlockIndex <= state.book.scrollIndex && state.book.scrollIndex <= it.lastBlockIndex }
                             .takeIf { it >= 0 }
                             ?: 0
                         onEvent(ReaderEvent.OnChangePage(page))
@@ -265,7 +266,7 @@ class ReaderModel @Inject constructor(
                         val pages = state.pages
                         if (pages.isEmpty()) return@withContext
                         val page = event.page.coerceIn(0, pages.lastIndex)
-                        val pageStart = pages.getOrNull(page)?.startTextIndex ?: 0
+                        val pageStart = pages.getOrNull(page)?.firstBlockIndex ?: 0
 
                         val (currentChapter, currentChapterProgress) = getChapterProgressUseCase(
                             pageStart,
@@ -300,10 +301,9 @@ class ReaderModel @Inject constructor(
                             _state.update { it.copy(pages = emptyList(), currentPage = 0) }
                             return@withContext
                         }
-                        // Restore the page that contains the saved scrollIndex.
                         val savedIndex = _state.value.book.scrollIndex
                         val targetPage = pages
-                            .indexOfFirst { it.startTextIndex <= savedIndex && savedIndex < it.endTextIndex }
+                            .indexOfFirst { it.firstBlockIndex <= savedIndex && savedIndex <= it.lastBlockIndex }
                             .takeIf { it >= 0 }
                             ?: 0
                         _state.update { it.copy(pages = pages, currentPage = targetPage) }
@@ -590,4 +590,4 @@ class ReaderModel @Inject constructor(
             this.value = function(this.value)
         }
     }
-}
+}

app/src/main/java/org/dueattendant149/bookshelf/presentation/reader/ReaderScreen.kt

@@ -25,7 +25,7 @@ import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.produceState
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.saveable.rememberSaveable
 import kotlinx.coroutines.Dispatchers
@@ -56,6 +56,8 @@ import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import kotlinx.parcelize.Parcelize
 import org.dueattendant149.bookshelf.core.helpers.calculateProgress
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderConstraints
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.presentation.navigator.Screen
 import org.dueattendant149.bookshelf.presentation.reader.model.ReaderColorEffects
 import org.dueattendant149.bookshelf.presentation.reader.model.ReaderProgressCount
@@ -239,8 +241,10 @@ data class ReaderScreen(val bookId: Int) : Screen, Parcelable {
         }
 
         // Recompute pages when text or relevant settings change.
-        val pages by produceState(
-            initialValue = emptyList<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>(),
+        val pagesState = remember { mutableStateOf(emptyList<ReaderPage2>()) }
+        val pages = pagesState.value
+
+        LaunchedEffect(
             state.value.text,
             readerPagination,
             settings.fontSize.value,
@@ -261,64 +265,70 @@ data class ReaderScreen(val bookId: Int) : Screen, Parcelable {
             configuration.screenWidthDp,
             configuration.screenHeightDp,
         ) {
-            value = withContext(Dispatchers.Default) {
-                if (!readerPagination || state.value.text.isEmpty()) {
-                    emptyList()
-                } else {
-                    val contentPaddingTopPx = with(density) {
-                        contentPadding.calculateTopPadding().toPx()
-                    }.toInt()
-                    val contentPaddingBottomPx = with(density) {
-                        contentPadding.calculateBottomPadding().toPx()
-                    }.toInt()
-                    val contentPaddingStartPx = with(density) {
-                        contentPadding.calculateStartPadding(layoutDirection).toPx()
-                    }.toInt()
-                    val contentPaddingEndPx = with(density) {
-                        contentPadding.calculateEndPadding(layoutDirection).toPx()
-                    }.toInt()
+            if (!readerPagination || state.value.text.isEmpty()) {
+                pagesState.value = emptyList()
+                return@LaunchedEffect
+            }
 
-                    val sidePaddingPx = with(density) { sidePadding.toPx() }.toInt()
-                    val verticalPaddingPx = with(density) { verticalPadding.toPx() }.toInt()
-                    val paragraphHeightPx = with(density) { paragraphHeight.toPx() }.toInt()
+            withContext(Dispatchers.Default) {
+                val contentPaddingTopPx = with(density) {
+                    contentPadding.calculateTopPadding().toPx()
+                }.toInt()
+                val contentPaddingBottomPx = with(density) {
+                    contentPadding.calculateBottomPadding().toPx()
+                }.toInt()
+                val contentPaddingStartPx = with(density) {
+                    contentPadding.calculateStartPadding(layoutDirection).toPx()
+                }.toInt()
+                val contentPaddingEndPx = with(density) {
+                    contentPadding.calculateEndPadding(layoutDirection).toPx()
+                }.toInt()
 
-                    val contentWidthPx = with(density) {
-                        configuration.screenWidthDp.dp.toPx()
-                    }.toInt() - contentPaddingStartPx - contentPaddingEndPx - sidePaddingPx * 2
+                val sidePaddingPx = with(density) { sidePadding.toPx() }.toInt()
+                val verticalPaddingPx = with(density) { verticalPadding.toPx() }.toInt()
+                val paragraphHeightPx = with(density) { paragraphHeight.toPx() }.toInt()
 
-                    val progressBarHeightPx = with(density) {
-                        (progressBarFontSize.toPx() * 1.5f + progressBarPadding.toPx() * 2).toInt()
-                    }
+                val contentWidthPx = with(density) {
+                    configuration.screenWidthDp.dp.toPx()
+                }.toInt() - contentPaddingStartPx - contentPaddingEndPx - sidePaddingPx * 2
 
-                    val contentHeightPx = with(density) {
-                        configuration.screenHeightDp.dp.toPx()
-                    }.toInt() - contentPaddingTopPx - contentPaddingBottomPx - verticalPaddingPx * 2 - paragraphHeightPx * 2 - progressBarHeightPx
-
-                    val imageMaxWidthPx = (contentWidthPx * imagesWidth).toInt()
-
-                    val style = readerParagraphTextStyle(
-                        fontFamily = settings.fontFamily.lastValue,
-                        fontThickness = settings.fontThickness.lastValue,
-                        fontStyle = if (settings.italic.lastValue) FontStyle.Italic else FontStyle.Normal,
-                        textAlignment = settings.textAlignment.lastValue,
-                        fontSize = settings.fontSize.lastValue.sp,
-                        lineHeight = (settings.fontSize.lastValue + settings.lineHeight.lastValue).sp,
-                        letterSpacing = (settings.letterSpacing.lastValue / 100f).em,
-                        paragraphIndentation = paragraphIndentation,
-                    )
-                    org.dueattendant149.bookshelf.domain.use_case.reader.PaginateReaderTextUseCase().invoke(
-                        text = state.value.text,
-                        contentWidthPx = contentWidthPx,
-                        contentHeightPx = contentHeightPx,
-                        paragraphStyle = style,
-                        chapterTitleAlignment = settings.chapterTitleAlignment.lastValue,
-                        paragraphSpacingPx = with(density) { paragraphHeight.toPx() }.toInt(),
-                        textMeasurer = textMeasurer,
-                        imageMaxWidthPx = imageMaxWidthPx,
-                        chapterExtraHeightPx = with(density) { 55.dp.toPx() }.toInt(),
-                        separatorHeightPx = with(density) { 3.dp.toPx() }.toInt(),
-                    )
+                val progressBarHeightPx = with(density) {
+                    (progressBarFontSize.toPx() * 1.5f + progressBarPadding.toPx() * 2).toInt()
                 }
+
+                val contentHeightPx = with(density) {
+                    configuration.screenHeightDp.dp.toPx()
+                }.toInt() - contentPaddingTopPx - contentPaddingBottomPx - verticalPaddingPx * 2 - paragraphHeightPx * 2 - progressBarHeightPx
+
+                val imageMaxWidthPx = (contentWidthPx * imagesWidth).toInt()
+
+                val style = readerParagraphTextStyle(
+                    fontFamily = settings.fontFamily.lastValue,
+                    fontThickness = settings.fontThickness.lastValue,
+                    fontStyle = if (settings.italic.lastValue) FontStyle.Italic else FontStyle.Normal,
+                    textAlignment = settings.textAlignment.lastValue,
+                    fontSize = settings.fontSize.lastValue.sp,
+                    lineHeight = (settings.fontSize.lastValue + settings.lineHeight.lastValue).sp,
+                    letterSpacing = (settings.letterSpacing.lastValue / 100f).em,
+                    paragraphIndentation = paragraphIndentation,
+                )
+
+                val constraints = ReaderConstraints(
+                    contentWidthPx = contentWidthPx,
+                    contentHeightPx = contentHeightPx,
+                    paragraphSpacingPx = paragraphHeightPx,
+                    separatorHeightPx = with(density) { 3.dp.toPx() }.toInt(),
+                )
+
+                pagesState.value = screenModel.paginationEngine.paginate(
+                    blocks = state.value.text,
+                    constraints = constraints,
+                    paragraphStyle = style,
+                    textMeasurer = textMeasurer,
+                    imageMaxWidthPx = imageMaxWidthPx,
+                    chapterExtraHeightPx = with(density) { 55.dp.toPx() }.toInt(),
+                    chapterTitleAlignment = settings.chapterTitleAlignment.lastValue.textAlignment,
+                ).pages
             }
         }
 

app/src/main/java/org/dueattendant149/bookshelf/presentation/reader/ReaderState.kt

@@ -12,16 +12,16 @@ import org.dueattendant149.bookshelf.core.BottomSheet
 import org.dueattendant149.bookshelf.core.Drawer
 import org.dueattendant149.bookshelf.core.ui.UIText
 import org.dueattendant149.bookshelf.domain.model.library.Book
-import org.dueattendant149.bookshelf.domain.model.reader.ReaderPage
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText.Chapter
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.presentation.reader.model.Checkpoint
 
 @Immutable
 data class ReaderState(
     val book: Book = Book.default,
     val text: List<ReaderText> = emptyList(),
-    val pages: List<ReaderPage> = emptyList(),
+    val pages: List<ReaderPage2> = emptyList(),
     val currentPage: Int = 0,
     val listState: LazyListState = LazyListState(),
 
@@ -37,4 +37,4 @@ data class ReaderState(
 
     val bottomSheet: BottomSheet? = null,
     val drawer: Drawer? = null
-)
+)

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderBottomBar.kt

@@ -42,6 +42,7 @@ import androidx.compose.ui.unit.dp
 import org.dueattendant149.bookshelf.R
 import org.dueattendant149.bookshelf.domain.model.library.Book
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.presentation.reader.ReaderEvent
 import org.dueattendant149.bookshelf.presentation.reader.model.Checkpoint
 import org.dueattendant149.bookshelf.ui.common.components.common.IconButton
@@ -62,7 +63,7 @@ fun ReaderBottomBar(
     checkpoints: List<Checkpoint>,
     bottomBarPadding: Dp,
     readerPagination: Boolean,
-    pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>,
+    pages: List<ReaderPage2>,
     currentPage: Int,
     onChangePage: (Int) -> Unit,
     restoreCheckpoint: (ReaderEvent.OnRestoreCheckpoint) -> Unit,
@@ -250,7 +251,7 @@ private fun ReaderBottomBarSlider(
     lockMenu: Boolean,
     listState: LazyListState,
     readerPagination: Boolean,
-    pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>,
+    pages: List<ReaderPage2>,
     currentPage: Int,
     onChangePage: (Int) -> Unit,
     scroll: (ReaderEvent.OnScroll) -> Unit,
@@ -274,7 +275,7 @@ private fun ReaderBottomBarSlider(
                 changeProgress(
                     ReaderEvent.OnChangeProgress(
                         progress = it,
-                        firstVisibleItemIndex = pages[page].startTextIndex,
+                        firstVisibleItemIndex = pages[page].firstBlockIndex,
                         firstVisibleItemOffset = 0
                     )
                 )

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderContent.kt

@@ -59,7 +59,7 @@ fun ReaderContent(
     showMenu: Boolean,
     lockMenu: Boolean,
     readerPagination: Boolean,
-    pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>,
+    pages: List<org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2>,
     currentPage: Int,
     onPageChanged: (Int) -> Unit,
     contentPadding: PaddingValues,

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderLayout.kt

@@ -56,7 +56,7 @@ import org.dueattendant149.bookshelf.ui.theme.model.HorizontalAlignment
 @Composable
 fun ReaderLayout(
     text: List<ReaderText>,
-    pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>,
+    pages: List<org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2>,
     currentPage: Int,
     readerPagination: Boolean,
     onPageChanged: (Int) -> Unit,
@@ -182,6 +182,7 @@ fun ReaderLayout(
         ) {
             if (readerPagination && pages.isNotEmpty()) {
                 ReaderPaginationLayout(
+                    text = text,
                     pages = pages,
                     currentPage = currentPage,
                     contentPadding = PaddingValues(0.dp),

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderPaginationLayout.kt

@@ -29,8 +29,8 @@ import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.TextUnit
 import androidx.compose.ui.unit.dp
 import kotlinx.coroutines.launch
-import org.dueattendant149.bookshelf.domain.model.reader.ReaderPage
 import org.dueattendant149.bookshelf.domain.model.reader.ReaderText
+import org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2
 import org.dueattendant149.bookshelf.presentation.reader.ReaderEvent
 import org.dueattendant149.bookshelf.presentation.reader.model.ReaderFontThickness
 import org.dueattendant149.bookshelf.presentation.reader.model.ReaderTextAlignment
@@ -43,7 +43,8 @@ import org.dueattendant149.bookshelf.ui.theme.model.HorizontalAlignment
 
 @Composable
 fun ReaderPaginationLayout(
-    pages: List<ReaderPage>,
+    text: List<ReaderText>,
+    pages: List<ReaderPage2>,
     currentPage: Int,
     contentPadding: PaddingValues,
     verticalPadding: Dp,
@@ -155,13 +156,15 @@ fun ReaderPaginationLayout(
                     .fillMaxWidth()
                     .weight(1f)
             ) { pageIndex ->
-                ReaderPageContent(
+                ReaderPageContent2(
                     page = pages[pageIndex],
+                    blocks = text,
                     verticalPadding = paragraphHeight,
                     sidePadding = sidePadding,
                     backgroundColor = backgroundColor,
                     fontColor = fontColor,
                     images = images,
+                    imagesCaptions = imagesCaptions,
                     imagesCornersRoundness = imagesCornersRoundness,
                     imagesAlignment = imagesAlignment,
                     imagesWidth = imagesWidth,
@@ -201,13 +204,15 @@ fun ReaderPaginationLayout(
 }
 
 @Composable
-private fun ReaderPageContent(
-    page: ReaderPage,
+private fun ReaderPageContent2(
+    page: ReaderPage2,
+    blocks: List<ReaderText>,
     verticalPadding: Dp,
     sidePadding: Dp,
     backgroundColor: Color,
     fontColor: Color,
     images: Boolean,
+    imagesCaptions: Boolean,
     imagesCornersRoundness: Dp,
     imagesAlignment: HorizontalAlignment,
     imagesWidth: Float,
@@ -237,15 +242,23 @@ private fun ReaderPageContent(
         verticalArrangement = Arrangement.spacedBy(verticalPadding, Alignment.Top),
         horizontalAlignment = horizontalAlignment
     ) {
-        page.items.forEachIndexed { index, entry ->
-            val previousEntry = page.items.getOrNull(index - 1)
+        page.spans.forEach { span ->
+            val block = blocks.getOrNull(span.blockIndex) ?: return@forEach
+
+            val displayBlock = when {
+                block is ReaderText.Text && !span.isAtomic ->
+                    ReaderText.Text(block.line.subSequence(span.startChar, span.endChar))
+                else -> block
+            }
+
+            val previousBlock = blocks.getOrNull(span.blockIndex - 1)
             when {
-                !images && (entry is ReaderText.Image || previousEntry is ReaderText.Image) -> return@forEachIndexed
-                !images && previousEntry is ReaderText.Image -> return@forEachIndexed
+                !images && (displayBlock is ReaderText.Image || previousBlock is ReaderText.Image) -> return@forEach
+                !imagesCaptions && previousBlock is ReaderText.Image -> return@forEach
                 else -> {
                     ReaderLayoutText(
                         showMenu = showMenu,
-                        entry = entry,
+                        entry = displayBlock,
                         imagesCornersRoundness = imagesCornersRoundness,
                         imagesAlignment = imagesAlignment,
                         imagesWidth = imagesWidth,

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderScaffold.kt

@@ -63,7 +63,7 @@ fun ReaderScaffold(
     showMenu: Boolean,
     lockMenu: Boolean,
     readerPagination: Boolean,
-    pages: List<org.dueattendant149.bookshelf.domain.model.reader.ReaderPage>,
+    pages: List<org.dueattendant149.bookshelf.domain.model.reader.pagination.ReaderPage2>,
     currentPage: Int,
     onPageChanged: (Int) -> Unit,
     contentPadding: PaddingValues,

app/src/main/java/org/dueattendant149/bookshelf/ui/reader/ReaderTextStyle.kt

@@ -53,28 +53,3 @@ fun readerParagraphTextStyle(
         platformStyle = PlatformTextStyle(includeFontPadding = includeFontPadding),
     )
 }
-
-/**
- * Builds a [TextStyle] for measuring chapter titles. It intentionally mirrors
- * the rendering style used by [ReaderLayoutTextChapter] as closely as possible
- * without accessing MaterialTheme at the call site.
- */
-fun readerChapterTextStyle(
-    nested: Boolean,
-    textAlignment: ReaderTextAlignment,
-    color: Color = Color.Unspecified,
-): TextStyle {
-    // Mirror Material3 headlineMedium / headlineSmall sizing. These are the
-    // default Material3 values; they may differ slightly from a customized
-    // theme, but they are accurate enough for pagination.
-    val fontSize = if (nested) 24.sp else 28.sp
-    val lineHeight = if (nested) 32.sp else 36.sp
-    return TextStyle(
-        fontSize = fontSize,
-        lineHeight = lineHeight,
-        fontWeight = FontWeight.Normal,
-        textAlign = textAlignment.textAlignment,
-        color = color,
-        platformStyle = PlatformTextStyle(includeFontPadding = false),
-    )
-}