Fix overlapping subtitles with negative line numbers

libraries/ui/src/main/java/androidx/media3/ui/CanvasSubtitleOutput.java

@@ -104,12 +104,36 @@ public void dispatchDraw(Canvas canvas) {
       return;
     }
 
+    // Track cumulative offsets for stacked cues to prevent overlapping.
+    // Bottom-stacked cues (negative line numbers) stack upward from the bottom.
+    // Top-stacked cues (positive line numbers) stack downward from the top.
+    int cumulativeBottomOffset = 0;
+    int cumulativeTopOffset = 0;
+
     int cueCount = cues.size();
     for (int i = 0; i < cueCount; i++) {
       Cue cue = cues.get(i);
       if (cue.verticalType != Cue.TYPE_UNSET) {
         cue = repositionVerticalCue(cue);
       }
+
+      // Determine if this cue is stacked from bottom or top based on line number.
+      // Cues with DIMEN_UNSET (no explicit position) default to bottom stacking.
+      boolean isBottomStackedCue =
+          cue.line == Cue.DIMEN_UNSET
+              || (cue.lineType == Cue.LINE_TYPE_NUMBER && cue.line < 0);
+      boolean isTopStackedCue =
+          cue.line != Cue.DIMEN_UNSET && cue.lineType == Cue.LINE_TYPE_NUMBER && cue.line >= 0;
+
+      // Adjust boundaries to account for previously drawn cues.
+      int adjustedTop = top;
+      int adjustedBottom = bottom;
+      if (isBottomStackedCue && cumulativeBottomOffset > 0) {
+        adjustedBottom = bottom - cumulativeBottomOffset;
+      } else if (isTopStackedCue && cumulativeTopOffset > 0) {
+        adjustedTop = top + cumulativeTopOffset;
+      }
+
       float cueTextSizePx =
           SubtitleViewUtils.resolveTextSize(
               cue.textSizeType, cue.textSize, rawViewHeight, viewHeightMinusPadding);
@@ -122,9 +146,16 @@ public void dispatchDraw(Canvas canvas) {
           bottomPaddingFraction,
           canvas,
           left,
-          top,
+          adjustedTop,
           right,
-          bottom);
+          adjustedBottom);
+
+      // Accumulate offset so subsequent cues don't overlap.
+      if (isBottomStackedCue) {
+        cumulativeBottomOffset += painter.getLastDrawnCueHeight();
+      } else if (isTopStackedCue) {
+        cumulativeTopOffset += painter.getLastDrawnCueHeight();
+      }
     }
   }
 

libraries/ui/src/main/java/androidx/media3/ui/SubtitlePainter.java

@@ -52,6 +52,9 @@
   /** Ratio of inner padding to font size. */
   private static final float INNER_PADDING_RATIO = 0.125f;
 
+  /** The height of the last drawn cue, or 0 if no cue was drawn. */
+  private int lastDrawnCueHeight;
+
   // Styled dimensions.
   private final float outlineWidth;
   private final float shadowRadius;
@@ -218,13 +221,27 @@ public void draw(
     if (isTextCue) {
       checkNotNull(cueText);
       setupTextLayout();
+      lastDrawnCueHeight = textLayout != null ? textLayout.getHeight() : 0;
     } else {
       checkNotNull(cueBitmap);
       setupBitmapLayout();
+      lastDrawnCueHeight = bitmapRect != null ? bitmapRect.height() : 0;
     }
     drawLayout(canvas, isTextCue);
   }
 
+  /**
+   * Returns the height of the last drawn cue.
+   *
+   * <p>This can be used to stack multiple cues without overlap by adjusting the drawing bounds for
+   * subsequent cues by this amount.
+   *
+   * @return The height of the last drawn cue in pixels, or 0 if no cue was drawn.
+   */
+  public int getLastDrawnCueHeight() {
+    return lastDrawnCueHeight;
+  }
+
   @RequiresNonNull("cueText")
   private void setupTextLayout() {
     SpannableStringBuilder cueText =

libraries/ui/src/test/java/androidx/media3/ui/CanvasSubtitleOutputTest.java

@@ -0,0 +1,273 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.media3.ui;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import android.content.Context;
+import android.graphics.Bitmap;
+import android.graphics.Canvas;
+import android.graphics.Color;
+import androidx.media3.common.text.Cue;
+import androidx.test.core.app.ApplicationProvider;
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+/** Tests for {@link CanvasSubtitleOutput}. */
+@RunWith(AndroidJUnit4.class)
+public class CanvasSubtitleOutputTest {
+
+  private static final int VIEW_WIDTH = 1920;
+  private static final int VIEW_HEIGHT = 1080;
+
+  private Context context;
+  private CanvasSubtitleOutput subtitleOutput;
+  private Canvas canvas;
+  private Bitmap bitmap;
+
+  @Before
+  public void setUp() {
+    context = ApplicationProvider.getApplicationContext();
+    subtitleOutput = new CanvasSubtitleOutput(context);
+    // Set up the view with a fixed size for consistent testing
+    subtitleOutput.layout(0, 0, VIEW_WIDTH, VIEW_HEIGHT);
+    bitmap = Bitmap.createBitmap(VIEW_WIDTH, VIEW_HEIGHT, Bitmap.Config.ARGB_8888);
+    canvas = new Canvas(bitmap);
+  }
+
+  @Test
+  public void dispatchDraw_emptyCues_doesNotCrash() {
+    subtitleOutput.update(
+        Collections.emptyList(),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_singleCue_renders() {
+    Cue cue = new Cue.Builder().setText("Single subtitle").build();
+
+    subtitleOutput.update(
+        Collections.singletonList(cue),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_overlappingCuesWithNegativeLineNumbers_rendersWithoutOverlap() {
+    // Simulate WebVTT overlapping cues that get assigned negative line numbers
+    // by WebvttSubtitle.getCues()
+    Cue cue1 =
+        new Cue.Builder()
+            .setText("First overlapping subtitle")
+            .setLine(-1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue cue2 =
+        new Cue.Builder()
+            .setText("Second overlapping subtitle")
+            .setLine(-2f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(cue1, cue2),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw and should render both cues
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_multipleOverlappingCues_rendersAll() {
+    // Test with 3+ overlapping cues (common in anime subtitles)
+    Cue cue1 =
+        new Cue.Builder()
+            .setText("Line 1")
+            .setLine(-1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue cue2 =
+        new Cue.Builder()
+            .setText("Line 2")
+            .setLine(-2f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue cue3 =
+        new Cue.Builder()
+            .setText("Line 3")
+            .setLine(-3f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(cue1, cue2, cue3),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_mixedCueTypes_rendersCorrectly() {
+    // Mix of cues with and without explicit line numbers
+    Cue cueWithLine =
+        new Cue.Builder()
+            .setText("Cue with line")
+            .setLine(-1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue cueWithoutLine = new Cue.Builder().setText("Cue without line").build();
+
+    subtitleOutput.update(
+        Arrays.asList(cueWithLine, cueWithoutLine),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_cuesWithFractionalLine_notAffectedByStacking() {
+    // Cues with LINE_TYPE_FRACTION should not be affected by the stacking logic
+    Cue cue1 =
+        new Cue.Builder()
+            .setText("Fractional line cue 1")
+            .setLine(0.9f, Cue.LINE_TYPE_FRACTION)
+            .build();
+    Cue cue2 =
+        new Cue.Builder()
+            .setText("Fractional line cue 2")
+            .setLine(0.8f, Cue.LINE_TYPE_FRACTION)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(cue1, cue2),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_cuesWithPositiveLineNumbers_stacksCorrectly() {
+    // Cues with positive line numbers (top-anchored) should stack downward from the top
+    Cue cue1 =
+        new Cue.Builder()
+            .setText("Top line cue 1")
+            .setLine(0f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue cue2 =
+        new Cue.Builder()
+            .setText("Top line cue 2")
+            .setLine(1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(cue1, cue2),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_mixedPositiveAndNegativeLineNumbers_stacksCorrectly() {
+    // Test with cues at both top and bottom of screen
+    Cue topCue =
+        new Cue.Builder()
+            .setText("Top subtitle")
+            .setLine(0f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue bottomCue =
+        new Cue.Builder()
+            .setText("Bottom subtitle")
+            .setLine(-1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(topCue, bottomCue),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+
+  @Test
+  public void dispatchDraw_multiLineCueWithOverlap_stacksCorrectly() {
+    // Test with a multi-line cue followed by another cue
+    // This is the core issue from the bug report
+    Cue multiLineCue =
+        new Cue.Builder()
+            .setText("This is a very long subtitle that will wrap to multiple lines")
+            .setLine(-1f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+    Cue secondCue =
+        new Cue.Builder()
+            .setText("Second cue")
+            .setLine(-2f, Cue.LINE_TYPE_NUMBER)
+            .setLineAnchor(Cue.ANCHOR_TYPE_START)
+            .build();
+
+    subtitleOutput.update(
+        Arrays.asList(multiLineCue, secondCue),
+        CaptionStyleCompat.DEFAULT,
+        /* textSize= */ 0.05f,
+        Cue.TEXT_SIZE_TYPE_FRACTIONAL,
+        /* bottomPaddingFraction= */ 0.08f);
+
+    // Should not throw
+    subtitleOutput.dispatchDraw(canvas);
+  }
+}