fix(CEA): Offset text CEA-608 that are out of viewport (#7024)

Author: avelad

lib/cea/cea608_memory.js

@@ -79,17 +79,29 @@ shaka.cea.Cea608Memory = class {
    * @return {?shaka.extern.ICaptionDecoder.ClosedCaption}
    */
   forceEmit(startTime, endTime) {
+    const Cea608Memory = shaka.cea.Cea608Memory;
     const stream = `CC${((this.fieldNum_<< 1) | this.channelNum_) + 1}`;
     const topLevelCue = new shaka.text.Cue(
         startTime, endTime, /* payload= */ '');
     topLevelCue.lineInterpretation =
         shaka.text.Cue.lineInterpretation.PERCENTAGE;
-    const line = shaka.cea.Cea608Memory.ROW_TO_LINE_CONVERSION_.get(this.row_);
+    let line = Cea608Memory.ROW_TO_LINE_CONVERSION_.get(this.row_);
     if (line) {
       topLevelCue.line = line;
     }
-    return shaka.cea.CeaUtils.getParsedCaption(
+    const ret = shaka.cea.CeaUtils.getParsedCaption(
         topLevelCue, stream, this.rows_, startTime, endTime);
+    // If the text and its lines are larger than what we can show on the
+    // screen, we move the lines up so that the text does not come out of the
+    // video.
+    if (ret && (this.row_ + ret.cue.nestedCues.length - 3) > 15) {
+      const newLinePosition = this.row_ + 3 - ret.cue.nestedCues.length;
+      line = Cea608Memory.ROW_TO_LINE_CONVERSION_.get(newLinePosition);
+      if (line) {
+        topLevelCue.line = line;
+      }
+    }
+    return ret;
   }
 
   /**