Fix bug where offscreen layers would erroneously draw black pixels (#55762)

Summary:
Pull Request resolved: #55762

On API <= 28, `clipWithAntiAliasing` uses `saveLayer` for anti-aliased border radius clipping. When a view is partially off-screen, the GPU only renders into the visible portion of the `saveLayer` buffer, leaving off-screen pixels as opaque black. These black pixels survive Porter-Duff compositing and appear as visible artifacts.

Adding `canvas.clipRect(0, 0, view.width, view.height)` before `saveLayer` forces HWUI to properly initialize the offscreen buffer relative to the GPU scissor. The clip is in local coordinates, so it stays correct across parent transform animations and scrolling offsets.

Changelog: [Internal]

Reviewed By: NickGerleman, jorge-cab

Differential Revision: D94447724

fbshipit-source-id: cb8470a00ca489a79779fe9b8e4f200c39f83284

Author: Abbondanzo

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/BackgroundStyleApplicator.kt

@@ -20,6 +20,7 @@ import android.os.Build
 import android.view.View
 import android.widget.ImageView
 import androidx.annotation.ColorInt
+import androidx.core.graphics.withClip
 import com.facebook.react.bridge.ReadableArray
 import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags
@@ -574,41 +575,49 @@ public object BackgroundStyleApplicator {
       paddingBoxPath: Path,
       drawContent: () -> Unit,
   ) {
-    // Save the layer for Porter-Duff compositing
-    val saveCount = canvas.saveLayer(0f, 0f, view.width.toFloat(), view.height.toFloat(), null)
-
-    // Draw the content first
-    drawContent()
-
-    val maskPaint = Paint(Paint.ANTI_ALIAS_FLAG)
-    maskPaint.style = Paint.Style.FILL
+    // Clip to the view's own bounds before saveLayer. On API <= 28 hardware-accelerated canvases,
+    // the window boundary is tracked by the GPU scissor but not reflected in the canvas clip stack.
+    // Without an explicit software clip, saveLayer may allocate a buffer with uninitialized pixels
+    // beyond the GPU scissor. Adding clipRect in the view's local coordinate space forces HWUI to
+    // include it in the clip stack, ensuring saveLayer properly constrains its buffer. This clip is
+    // stable across parent transform animations since it's in the view's own coordinate space.
+    canvas.withClip(0, 0, view.width, view.height) {
+      // Save the layer for Porter-Duff compositing
+      val saveCount = canvas.saveLayer(0f, 0f, view.width.toFloat(), view.height.toFloat(), null)
+
+      // Draw the content first
+      drawContent()
+
+      val maskPaint = Paint(Paint.ANTI_ALIAS_FLAG)
+      maskPaint.style = Paint.Style.FILL
+
+      // Transparent pixels with INVERSE_WINDING only works on API 28
+      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
+        maskPaint.xfermode = PorterDuffXfermode(PorterDuff.Mode.DST_IN)
+        maskPaint.color = Color.TRANSPARENT
+        paddingBoxPath.setFillType(Path.FillType.INVERSE_WINDING)
+        canvas.drawPath(paddingBoxPath, maskPaint)
+      } else {
+        // API < 28: Use a nested saveLayer with DST_IN compositing to mask content to the
+        // padding box path. EVEN_ODD fill + DST_OUT has rendering bugs on API 24's hardware
+        // renderer, so we avoid that technique. Instead, draw the mask shape into a separate
+        // layer; when restored with DST_IN, content is preserved only where the mask is opaque.
+        val dstInPaint = Paint()
+        dstInPaint.xfermode = PorterDuffXfermode(PorterDuff.Mode.DST_IN)
+        val maskSave =
+            canvas.saveLayer(0f, 0f, view.width.toFloat(), view.height.toFloat(), dstInPaint)
+        // Clear the layer to ensure it starts fully transparent. On API 24, saveLayer may not
+        // initialize the buffer to transparent, causing DST_IN to see non-zero alpha everywhere.
+        canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)
+        maskPaint.xfermode = null
+        maskPaint.color = Color.BLACK
+        canvas.drawPath(paddingBoxPath, maskPaint)
+        canvas.restoreToCount(maskSave)
+      }
 
-    // Transparent pixels with INVERSE_WINDING only works on API 28
-    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
-      maskPaint.xfermode = PorterDuffXfermode(PorterDuff.Mode.DST_IN)
-      maskPaint.color = Color.TRANSPARENT
-      paddingBoxPath.setFillType(Path.FillType.INVERSE_WINDING)
-      canvas.drawPath(paddingBoxPath, maskPaint)
-    } else {
-      // API < 28: Use a nested saveLayer with DST_IN compositing to mask content to the
-      // padding box path. EVEN_ODD fill + DST_OUT has rendering bugs on API 24's hardware
-      // renderer, so we avoid that technique. Instead, draw the mask shape into a separate
-      // layer; when restored with DST_IN, content is preserved only where the mask is opaque.
-      val dstInPaint = Paint()
-      dstInPaint.xfermode = PorterDuffXfermode(PorterDuff.Mode.DST_IN)
-      val maskSave =
-          canvas.saveLayer(0f, 0f, view.width.toFloat(), view.height.toFloat(), dstInPaint)
-      // Clear the layer to ensure it starts fully transparent. On API 24, saveLayer may not
-      // initialize the buffer to transparent, causing DST_IN to see non-zero alpha everywhere.
-      canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)
-      maskPaint.xfermode = null
-      maskPaint.color = Color.BLACK
-      canvas.drawPath(paddingBoxPath, maskPaint)
-      canvas.restoreToCount(maskSave)
+      // Restore the layer
+      canvas.restoreToCount(saveCount)
     }
-
-    // Restore the layer
-    canvas.restoreToCount(saveCount)
   }
 
   /**