Fix PixelCopy snapshot for partially off-screen views (facebook#56608)

Summary:

[Internal] [Fixed] - Fix PixelCopy snapshot for partially off-screen 

PixelCopy captures only the visible portion of the window surface.
When a view is partially off-screen, the capture rect extends beyond
the window bounds, resulting in a bitmap where only the visible
region has content. This partial bitmap then gets stretched to fill
the full-size pseudo-element, causing visual distortion.

Fix by clamping the PixelCopy rect to the window bounds and
compositing the clamped capture into a full-size bitmap at the
correct offset. The off-screen portions remain transparent instead
of being stretched.

If the view is entirely off-screen, skip capture entirely — the
pseudo-element will have no snapshot applied (didMountItems skips
tags without a captured bitmap).

Differential Revision: D102360642

Author: zeyap

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/ViewTransitionSnapshotManager.kt

@@ -103,28 +103,54 @@ internal class ViewTransitionSnapshotManager(
 
   @RequiresApi(Build.VERSION_CODES.O)
   private fun captureHardwareBitmap(view: View, reactTag: Int, window: Window) {
-    val bitmap = createBitmap(view.width, view.height)
     val location = IntArray(2)
     view.getLocationInWindow(location)
-    val rect = Rect(location[0], location[1], location[0] + view.width, location[1] + view.height)
+
+    // The view's rect in window coordinates.
+    val viewRect =
+        Rect(location[0], location[1], location[0] + view.width, location[1] + view.height)
+
+    // Clamp to window bounds — PixelCopy only captures what's visible on the
+    // window surface. Without clamping, off-screen portions are black/empty
+    // and the partial result gets stretched to fill the pseudo-element.
+    val windowWidth = window.decorView.width
+    val windowHeight = window.decorView.height
+    val clampedRect =
+        Rect(
+            viewRect.left.coerceAtLeast(0),
+            viewRect.top.coerceAtLeast(0),
+            viewRect.right.coerceAtMost(windowWidth),
+            viewRect.bottom.coerceAtMost(windowHeight),
+        )
+
+    if (clampedRect.isEmpty) {
+      // Entirely off-screen — nothing to capture.
+      return
+    }
+
+    val clampedBitmap = createBitmap(clampedRect.width(), clampedRect.height())
+    // Offset of the clamped region within the full view.
+    val offsetX = clampedRect.left - viewRect.left
+    val offsetY = clampedRect.top - viewRect.top
+
     // PixelCopy callback is posted to mainHandler, so onBitmapCaptured may run after
     // setViewSnapshot has already recorded the target tag for this source tag.
     try {
       PixelCopy.request(
           window,
-          rect,
-          bitmap,
+          clampedRect,
+          clampedBitmap,
           { copyResult ->
             if (copyResult == PixelCopy.SUCCESS) {
-              val hwBitmap = bitmap.copy(Bitmap.Config.HARDWARE, false)
-              if (hwBitmap != null) {
-                bitmap.recycle()
-                onBitmapCaptured(reactTag, hwBitmap)
-              } else {
-                onBitmapCaptured(reactTag, bitmap)
-              }
+              // Compose the clamped capture into a full-size bitmap at the
+              // correct offset so it aligns with the pseudo-element's bounds.
+              val fullBitmap = createBitmap(view.width, view.height)
+              Canvas(fullBitmap)
+                  .drawBitmap(clampedBitmap, offsetX.toFloat(), offsetY.toFloat(), null)
+              clampedBitmap.recycle()
+              onBitmapCaptured(reactTag, fullBitmap)
             } else {
-              bitmap.recycle()
+              clampedBitmap.recycle()
               onBitmapCaptured(reactTag, captureSoftwareBitmap(view))
             }
           },
@@ -133,7 +159,7 @@ internal class ViewTransitionSnapshotManager(
     } catch (e: IllegalArgumentException) {
       // Window surface may have been destroyed (e.g., device idle/sleep).
       // Fall back to software rendering.
-      bitmap.recycle()
+      clampedBitmap.recycle()
       onBitmapCaptured(reactTag, captureSoftwareBitmap(view))
     }
   }