Fix a bug with TTML font styling that displays empty lines.

If the caption line has no text (empty line or only line break), we should not
display its background.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196823319

RELEASENOTES.md

@@ -20,8 +20,11 @@
   * Fix playback of livestreams with EXT-X-PROGRAM-DATE-TIME tags
   * Fix playback of livestreams with EXT-X-PROGRAM-DATE-TIME tags
     ([#4239](https://github.com/google/ExoPlayer/issues/4239)).
     ([#4239](https://github.com/google/ExoPlayer/issues/4239)).
 * Caption:
 * Caption:
-  * Fix a TTML styling issue when there are multiple regions displayed at the
-    same time that can make text size of each region much smaller than defined.
+  * TTML:
+    * Fix a styling issue when there are multiple regions displayed at the same
+      time that can make text size of each region much smaller than defined.
+    * Fix an issue when the caption line has no text (empty line or only line
+      break), and the line's background is still displayed.
 
 
 ### 2.8.0 ###
 ### 2.8.0 ###
 
 

library/ui/src/main/java/com/google/android/exoplayer2/ui/SubtitlePainter.java

@@ -372,12 +372,22 @@ import com.google.android.exoplayer2.util.Util;
       float previousBottom = layout.getLineTop(0);
       float previousBottom = layout.getLineTop(0);
       int lineCount = layout.getLineCount();
       int lineCount = layout.getLineCount();
       for (int i = 0; i < lineCount; i++) {
       for (int i = 0; i < lineCount; i++) {
-        lineBounds.left = layout.getLineLeft(i) - textPaddingX;
-        lineBounds.right = layout.getLineRight(i) + textPaddingX;
+        float lineTextBoundLeft = layout.getLineLeft(i);
+        float lineTextBoundRight = layout.getLineRight(i);
+        lineBounds.left = lineTextBoundLeft - textPaddingX;
+        lineBounds.right = lineTextBoundRight + textPaddingX;
         lineBounds.top = previousBottom;
         lineBounds.top = previousBottom;
         lineBounds.bottom = layout.getLineBottom(i);
         lineBounds.bottom = layout.getLineBottom(i);
         previousBottom = lineBounds.bottom;
         previousBottom = lineBounds.bottom;
-        canvas.drawRoundRect(lineBounds, cornerRadius, cornerRadius, paint);
+        float lineTextWidth = lineTextBoundRight - lineTextBoundLeft;
+        if (lineTextWidth > 0) {
+          // Do not draw a line's background color if it has no text.
+          // For some reason, calculating the width manually is more reliable than
+          // layout.getLineWidth().
+          // Sometimes, lineTextBoundRight == lineTextBoundLeft, and layout.getLineWidth() still
+          // returns non-zero value.
+          canvas.drawRoundRect(lineBounds, cornerRadius, cornerRadius, paint);
+        }
       }
       }
     }
     }