fix(android): track textAlignVertical in custom decoration draw offset

ReactTextView.onDraw paints CanvasEffectSpan decorations (underline,
strikethrough) in their own pass, translating by getExtendedPaddingTop()
only. That matches the default Gravity.TOP, but when textAlignVertical is
center or bottom and the text is shorter than the view, super.onDraw shifts
the glyphs down by a gravity offset the span draws were missing, so the
decoration rendered at the top of the box while the text sat centered or at
the bottom.

The offset (mirroring the private TextView.getVerticalOffset()) is now
computed by a package-private verticalGravityOffset() helper and applied to
both the onPreDraw and onDraw translate passes.

## Changelog:

[ANDROID] [FIXED] - Custom text decorations track `textAlignVertical`

## Test Plan:

ReactTextViewTest covers verticalGravityOffset across top, center, bottom,
exact-fit, and overflow cases. Visually: a fixed-height <Text> with
textAlignVertical "center" (and "bottom"), textDecorationLine "underline",
and a distinct textDecorationColor inside a taller parent now renders the
decoration flush under the glyphs; previously it stayed pinned to the top of
the box while the text moved. Verified on Android API 36.

Author: quantizor

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/text/ReactTextView.java

@@ -219,8 +219,14 @@ protected void onDraw(Canvas canvas) {
           CanvasEffectSpan[] drawSpans =
               spanned.getSpans(0, spanned.length(), CanvasEffectSpan.class);
           if (drawSpans.length > 0) {
+            int voffsetText =
+                verticalGravityOffset(
+                    getGravity(),
+                    getMeasuredHeight() - getExtendedPaddingTop() - getExtendedPaddingBottom(),
+                    layout.getHeight());
+
             canvas.save();
-            canvas.translate(getCompoundPaddingLeft(), getExtendedPaddingTop());
+            canvas.translate(getCompoundPaddingLeft(), getExtendedPaddingTop() + voffsetText);
             for (CanvasEffectSpan span : drawSpans) {
               int start = spanned.getSpanStart(span);
               int end = spanned.getSpanEnd(span);
@@ -231,7 +237,7 @@ protected void onDraw(Canvas canvas) {
             super.onDraw(canvas);
 
             canvas.save();
-            canvas.translate(getCompoundPaddingLeft(), getExtendedPaddingTop());
+            canvas.translate(getCompoundPaddingLeft(), getExtendedPaddingTop() + voffsetText);
             for (CanvasEffectSpan span : drawSpans) {
               int start = spanned.getSpanStart(span);
               int end = spanned.getSpanEnd(span);
@@ -413,6 +419,20 @@ public boolean hasOverlappingRendering() {
     setGravity((getGravity() & ~Gravity.VERTICAL_GRAVITY_MASK) | gravityVertical);
   }
 
+  /**
+   * The vertical shift `super.onDraw` applies to glyphs for the current gravity, mirroring the
+   * private {@code TextView.getVerticalOffset()}. {@link CanvasEffectSpan} decorations paint in a
+   * separate pass and must add the same offset so they track the text. Returns 0 for {@code TOP}
+   * gravity and whenever the text fills or overflows the box (no room to shift).
+   */
+  /* package */ static int verticalGravityOffset(int gravity, int boxHeight, int textHeight) {
+    int vertical = gravity & Gravity.VERTICAL_GRAVITY_MASK;
+    if (vertical == Gravity.TOP || textHeight >= boxHeight) {
+      return 0;
+    }
+    return (vertical == Gravity.BOTTOM) ? (boxHeight - textHeight) : (boxHeight - textHeight) / 2;
+  }
+
   public void setNumberOfLines(int numberOfLines) {
     mNumberOfLines = numberOfLines == 0 ? ViewDefaults.NUMBER_OF_LINES : numberOfLines;
     setMaxLines(mNumberOfLines);

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/views/text/ReactTextViewTest.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.views.text
+
+import android.view.Gravity
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+
+/**
+ * Covers [ReactTextView.verticalGravityOffset], the offset that keeps CanvasEffectSpan decorations
+ * (underline, strikethrough) tracking the glyphs when `textAlignVertical` shifts the text within a
+ * taller box.
+ */
+@RunWith(RobolectricTestRunner::class)
+class ReactTextViewTest {
+
+  @Test
+  fun topGravityNeverShifts() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.TOP, 200, 40)).isEqualTo(0)
+  }
+
+  @Test
+  fun centerGravityShiftsByHalfTheSlack() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.CENTER_VERTICAL, 200, 40)).isEqualTo(80)
+  }
+
+  @Test
+  fun bottomGravityShiftsByFullSlack() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.BOTTOM, 200, 40)).isEqualTo(160)
+  }
+
+  @Test
+  fun centerGravityFloorsOddSlack() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.CENTER_VERTICAL, 101, 40)).isEqualTo(30)
+  }
+
+  @Test
+  fun fullBoxDoesNotShift() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.CENTER_VERTICAL, 200, 200)).isEqualTo(0)
+  }
+
+  @Test
+  fun overflowDoesNotShift() {
+    assertThat(ReactTextView.verticalGravityOffset(Gravity.BOTTOM, 200, 260)).isEqualTo(0)
+  }
+}