Read only the drawn child's mix-blend-mode in Android drawChild (#59016)

Summary:
`ReactViewGroup.drawChild` and `ReactRootView.drawChild` call `BlendModeHelper.needsIsolatedLayer(this)` before they read the drawn child's `mix_blend_mode` tag. `needsIsolatedLayer` reads that tag on every child of the group, and `dispatchDraw` calls `drawChild` once per child, so recording the display list of a group with N children reads N² tags. When no child sets a blend mode — the common case — `any` never short-circuits, so the whole scan runs for every child of every group that records.

I found it profiling flings over a grid of a few hundred views on two release builds: sampling the main thread, `needsIsolatedLayer` under `drawChild` was 37% of display-list recording on a Snapdragon 845 (Android 10) and 46% on a Snapdragon 8 Gen 2 (Android 14).

The guard adds nothing to the decision `drawChild` makes. A child that carries the tag makes `needsIsolatedLayer` true by definition, and a child without it draws with no layer whatever the guard says. So both overrides now read the drawn child's own tag. `draw()` keeps its group-wide check, which decides whether the whole group renders offscreen and runs once per recording.

I searched the tracker for `needsIsolatedLayer`, `BlendModeHelper`, `mix_blend_mode` and `drawChild` and found no existing report or PR.

## Changelog:

[ANDROID] [FIXED] - Drawing a view group no longer reads every sibling's mix-blend-mode for each child it draws

Pull Request resolved: #59016

Test Plan:
- `./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest --tests "com.facebook.react.views.view.ReactViewGroupTest" -Preact.internal.useHermesStable=true`: on `main` the new `drawing a child reads no sibling's blend mode` test fails with `expected:<[3]> but was:<[21]>` — each child of a 20-child group has its tag read 21 times, against 3 in a 2-child group; with the fix both groups read each child equally often, and all 19 tests in the class pass.
- The new `a child with a blend mode draws into a layer with that blend mode` test records `[null, MULTIPLY]` — the group's isolated layer, then the child's — before and after the change; with the child's layer skipped it fails with `Expecting actual: [null] to contain exactly (and in same order): [null, MULTIPLY]`.
- `./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest -PenableWarningsAsErrors=true -Preact.internal.useHermesStable=true`: 707 tests, 0 failures.
- `node ./scripts/format-kotlin.js --check` and `node ./scripts/format-java.js --check` pass.
- The same change on 0.86.3, in an app with a method trace over those flings: `needsIsolatedLayer` ran 1,608 times for 1,400 `drawChild` calls before and 182 times for 3,234 after, every one of them from `draw()`. A `mixBlendMode: 'multiply'` view in the same app still blends.
- Not run: the RNTester build and Android lint (`build_android`).

Reviewed By: andrewdacenko

Differential Revision: D124965507

Pulled By: javache

fbshipit-source-id: e0155f6de592f9d14e8cb17a6b092b5fda711d04

Author: YevheniiKotyrlo

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -8,7 +8,6 @@
 package com.facebook.react;
 
 import static com.facebook.infer.annotation.ThreadConfined.UI;
-import static com.facebook.react.uimanager.BlendModeHelper.needsIsolatedLayer;
 import static com.facebook.react.uimanager.common.UIManagerType.FABRIC;
 import static com.facebook.systrace.Systrace.TRACE_TAG_REACT;
 
@@ -308,7 +307,7 @@ protected void dispatchDraw(Canvas canvas) {
   protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
 
     BlendMode mixBlendMode = null;
-    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q && needsIsolatedLayer(this)) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
       mixBlendMode = (BlendMode) child.getTag(R.id.mix_blend_mode);
       if (mixBlendMode != null) {
         Paint p = new Paint();

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/view/ReactViewGroup.kt

@@ -1003,7 +1003,7 @@ public open class ReactViewGroup public constructor(context: Context?) :
     }
 
     var mixBlendMode: BlendMode? = null
-    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q && needsIsolatedLayer(this)) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
       mixBlendMode = child.getTag(R.id.mix_blend_mode) as? BlendMode
       if (mixBlendMode != null) {
         val p = Paint()

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/views/view/ReactViewGroupTest.kt

@@ -9,9 +9,13 @@ package com.facebook.react.views.view
 
 import android.app.Activity
 import android.content.Context
+import android.graphics.BlendMode
+import android.graphics.Canvas
+import android.graphics.Paint
 import android.view.View
 import android.view.ViewGroup
 import android.widget.FrameLayout
+import com.facebook.react.R
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsDefaults
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests
@@ -67,6 +71,32 @@ class ReactViewGroupTest {
     assertThat(rvg.childCount).isEqualTo(20)
   }
 
+  @Test
+  fun `mix blend mode - drawing a child reads no sibling's blend mode`() {
+    fun blendModeReadsPerChild(childCount: Int): Int {
+      val rvg = ReactViewGroup(context)
+      val children = List(childCount) { BlendModeReadCountingView(context) }
+      children.forEach { rvg.addView(it) }
+      rvg.layout(0, 0, 100, 100)
+      rvg.draw(Canvas())
+      return children.maxOf { it.blendModeReads }
+    }
+
+    assertThat(blendModeReadsPerChild(20)).isEqualTo(blendModeReadsPerChild(2))
+  }
+
+  @Test
+  fun `mix blend mode - a child with a blend mode draws into a layer with that blend mode`() {
+    val rvg = ReactViewGroup(context)
+    rvg.addView(View(context))
+    rvg.addView(View(context).apply { setTag(R.id.mix_blend_mode, BlendMode.MULTIPLY) })
+    rvg.addView(View(context))
+    rvg.layout(0, 0, 100, 100)
+    val canvas = LayerRecordingCanvas()
+    rvg.draw(canvas)
+    assertThat(canvas.layerBlendModes).containsExactly(null, BlendMode.MULTIPLY)
+  }
+
   @Test
   fun `auto offscreen compositing - true when flag on, alpha lt 1, and elevated descendant`() {
     overrideAutoOffscreenFlag(true)
@@ -294,3 +324,23 @@ class TestView(context: Context, yPos: Int) : View(context) {
 class TestParent(context: Context) : ViewGroup(context) {
   override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) = Unit
 }
+
+private class BlendModeReadCountingView(context: Context) : View(context) {
+  var blendModeReads = 0
+
+  override fun getTag(key: Int): Any? {
+    if (key == R.id.mix_blend_mode) {
+      blendModeReads++
+    }
+    return super.getTag(key)
+  }
+}
+
+private class LayerRecordingCanvas : Canvas() {
+  val layerBlendModes = mutableListOf<BlendMode?>()
+
+  override fun saveLayer(left: Float, top: Float, right: Float, bottom: Float, paint: Paint?): Int {
+    layerBlendModes.add(paint?.blendMode)
+    return super.saveLayer(left, top, right, bottom, paint)
+  }
+}