feat(pdf-viewer): implement pan lock to disable horizontal movement (#9)

- Add `isScrollLocked` state to `PdfViewerScreen` to toggle panning behavior.
- Replace "Import SVG" debug icon with a Lock/Unlock toggle in the Top App Bar.
- Update `PdfVerticalReader` and `PdfPageComposable` to strictly zero out X-axis pan deltas and fling velocities when locked.
- Maintain zoom and vertical scrolling functionality while panning is locked.

app/src/main/java/com/aryan/reader/pdf/PdfPageComposable.kt

@@ -17,6 +17,7 @@
  *
  * mail: epistemereader@gmail.com
  */
+// PdfPageComposable
 @file:Suppress(
     "RemoveRedundantQualifierName", "COMPOSE_APPLIER_CALL_MISMATCH", "UnusedVariable", "unused"
 )
@@ -434,6 +435,7 @@ internal fun PdfPageComposable(
     onTextBoxDragEnd: () -> Unit = {},
     onDragPageTurn: (Int) -> Unit = {},
     draggingBoxId: String? = null,
+    isScrollLocked: Boolean = false,
     isVisible: Boolean = true,
 ) {
     SideEffect { Timber.tag("PdfDrawPerf").v("PdfPageComposable Recompose: Page $pageIndex") }
@@ -2422,7 +2424,8 @@ internal fun PdfPageComposable(
                 isZoomEnabled,
                 isVerticalScroll,
                 isEditMode,
-                onTwoFingerSwipe
+                onTwoFingerSwipe,
+                isScrollLocked
             ) {
                 if (!isZoomEnabled || isVerticalScroll || actualBitmapWidthPx == 0 || activeDraggingHandle != null) return@pointerInput
 
@@ -2453,7 +2456,8 @@ internal fun PdfPageComposable(
                         }
 
                         if (!canceled) {
-                            val panChange = event.calculatePan()
+                            val rawPanChange = event.calculatePan()
+                            val panChange = if (isScrollLocked) Offset(0f, rawPanChange.y) else rawPanChange
                             val zoomChange = event.calculateZoom()
 
                             if (scale > 1f) {
@@ -2622,13 +2626,15 @@ internal fun PdfPageComposable(
                         coroutineScope.launch {
                             coroutineScope {
                                 launch {
-                                    Animatable(startX).animateDecay(
+                                    if (!isScrollLocked) {
-                                        velocity.x, decay
+                                        Animatable(startX).animateDecay(
-                                    ) {
+                                            velocity.x, decay
-                                        val newX = value.coerceIn(
+                                        ) {
-                                            -maxOffsetX, maxOffsetX
+                                            val newX = value.coerceIn(
-                                        )
+                                                -maxOffsetX, maxOffsetX
-                                        offset = offset.copy(x = newX)
+                                            )
+                                            offset = offset.copy(x = newX)
+                                        }
                                     }
                                 }
                                 launch {

app/src/main/java/com/aryan/reader/pdf/PdfVerticalReader.kt

@@ -214,6 +214,7 @@ internal fun PdfVerticalReader(
     onTextBoxMoved: (String, Int, Rect) -> Unit = { _, _, _ -> },
     isAutoScrollPlaying: Boolean = false,
     isAutoScrollTempPaused: Boolean = false,
+    isScrollLocked: Boolean = false,
     autoScrollSpeed: Float = 1.0f,
     onInteractionListener: () -> Unit = {}
 ) {
@@ -837,7 +838,7 @@ internal fun PdfVerticalReader(
                     })
                 }
 
-                .pointerInput(totalDocHeight, isEditMode, selectedTool) {
+                .pointerInput(totalDocHeight, isEditMode, selectedTool, isScrollLocked) {
                     val tracker = VelocityTracker()
                     val decay = exponentialDecay<Float>()
                     val touchSlop = viewConfiguration.touchSlop
@@ -916,7 +917,9 @@ internal fun PdfVerticalReader(
                                 }
 
                                 val zoomChange = event.calculateZoom()
-                                val panChange = event.calculatePan()
+                                val rawPanChange = event.calculatePan()
+                                val panChange = if (isScrollLocked) Offset(0f, rawPanChange.y) else rawPanChange
+
                                 val centroid = event.calculateCentroid(useCurrent = false)
                                 val panMagnitude = panChange.getDistance()
                                 val currentCentroidSize = event.calculateCentroidSize(
@@ -1065,7 +1068,7 @@ internal fun PdfVerticalReader(
                                     coroutineScope {
                                         launch {
                                             val rawX = velocity.x * flingSensitivity
-                                            val flingX = if (abs(rawX) > minFlingVelocity) rawX
+                                            val flingX = if (abs(rawX) > minFlingVelocity && !isScrollLocked) rawX
                                             else 0f
 
                                             if (flingX != 0f) panXAnimatable.animateDecay(

app/src/main/java/com/aryan/reader/pdf/PdfViewerScreen.kt

@@ -102,6 +102,8 @@ import androidx.compose.material.icons.filled.FullscreenExit
 import androidx.compose.material.icons.filled.Info
 import androidx.compose.material.icons.filled.KeyboardArrowDown
 import androidx.compose.material.icons.filled.KeyboardArrowUp
+import androidx.compose.material.icons.filled.Lock
+import androidx.compose.material.icons.filled.LockOpen
 import androidx.compose.material.icons.filled.Menu
 import androidx.compose.material.icons.filled.MoreVert
 import androidx.compose.material.icons.filled.Save
@@ -588,6 +590,7 @@ fun PdfViewerScreen(
     var showBars by remember { mutableStateOf(true) }
     var isFullScreen by remember { mutableStateOf(false) }
     var documentPassword by remember { mutableStateOf<String?>(null) }
+    var isScrollLocked by remember { mutableStateOf(false) }
     var showPasswordDialog by remember { mutableStateOf(false) }
     var isPasswordError by remember { mutableStateOf(false) }
     LocalView.current
@@ -3208,6 +3211,7 @@ fun PdfViewerScreen(
                                                 virtualPage = virtualPage,
                                                 totalPages = totalDisplayPages,
                                                 isDarkMode = isPdfDarkMode,
+                                                isScrollLocked = isScrollLocked,
                                                 onScaleChanged = { newScale ->
                                                     if (pagerState.currentPage == pageIndex) {
                                                         currentPageScale = newScale
@@ -3597,6 +3601,7 @@ fun PdfViewerScreen(
                                             state = verticalReaderState,
                                             pdfDocument = docHolder,
                                             isDarkMode = isPdfDarkMode,
+                                            isScrollLocked = isScrollLocked,
                                             totalPages = totalDisplayPages,
                                             pageAspectRatios = ratiosHolder,
                                             virtualPages = virtualPages,
@@ -4016,6 +4021,14 @@ fun PdfViewerScreen(
                                     )
                                 }
 
+                                IconButton(onClick = { isScrollLocked = !isScrollLocked }) {
+                                    Icon(
+                                        imageVector = if (isScrollLocked) Icons.Default.Lock else Icons.Default.LockOpen,
+                                        contentDescription = if (isScrollLocked) "Unlock Panning" else "Lock Panning",
+                                        tint = MaterialTheme.colorScheme.onSurfaceVariant
+                                    )
+                                }
+
                                 IconButton(onClick = { isFullScreen = true }) {
                                     Icon(
                                         imageVector = Icons.Default.Fullscreen,
@@ -4066,29 +4079,6 @@ fun PdfViewerScreen(
                                             tint = Color(0xFFE91E63)
                                         )
                                     }
-
-                                    IconButton(onClick = {
-                                        val page = if (displayMode == DisplayMode.PAGINATION) pagerState.currentPage else verticalReaderState.currentPage
-                                        val demoAnnos = DemoAnnotationGenerator.generateDemoAnnotations(page)
-
-                                        val existing = allAnnotations[page] ?: emptyList()
-                                        allAnnotations = allAnnotations + (page to (existing + demoAnnos))
-
-                                        demoAnnos.forEach { annot ->
-                                            undoStack.add(HistoryAction.Add(page, annot))
-                                        }
-                                        redoStack.clear()
-
-                                        coroutineScope.launch {
-                                            snackbarHostState.showSnackbar("Demo annotations injected!")
-                                        }
-                                    }) {
-                                        Icon(
-                                            imageVector = Icons.Default.Edit,
-                                            contentDescription = "Inject Demo Annotations",
-                                            tint = Color(0xFF2E7D32)
-                                        )
-                                    }
                                 }
 
                                 Box {