Document calls to the "rules for updating the text track rendering"

Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 3192 matching lines @@
     if (!m_player || !hasClosedCaptions())
         return;
 
     m_closedCaptionsVisible = closedCaptionVisible;
 
     markCaptionAndSubtitleTracksAsUnconfigured();
     m_processingPreferenceChange = true;
     honorUserPreferencesForAutomaticTextTrackSelection();
     m_processingPreferenceChange = false;
 
+    // As track visibility changed while m_processingPreferenceChange was set,
+    // there was no call to updateTextTrackDisplay(). This call is not in the
+    // spec, see the note in configureTextTrackDisplay().
     updateTextTrackDisplay();
 }
 
 void HTMLMediaElement::markCaptionAndSubtitleTracksAsUnconfigured()
 {
     if (!m_textTracks)
         return;
 
     // Mark all tracks as not "configured" so that
     // honorUserPreferencesForAutomaticTextTrackSelection() will reconsider
@@ skipping 118 matching lines @@
     m_haveVisibleTextTrack = haveVisibleTextTrack;
     m_closedCaptionsVisible = m_haveVisibleTextTrack;
 
     if (!m_haveVisibleTextTrack && !mediaControls())
         return;
 
     ensureMediaControls();
     mediaControls()->changedClosedCaptionsVisibility();
 
     cueTimeline().updateActiveCues(currentTime());
+
+    // Note: The "time marches on" algorithm (updateActiveCues) runs the "rules
+    // for updating the text track rendering" (updateTextTrackDisplay) only for
+    // "affected tracks", i.e. tracks where the the active cues have changed.
+    // This misses cues in tracks that changed mode between hidden and showing.
+    // This appears to be a spec bug, which we work around here:
+    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=28236
     updateTextTrackDisplay();
 }
 
 void* HTMLMediaElement::preDispatchEventHandler(Event* event)
 {
     if (event && event->type() == EventTypeNames::webkitfullscreenchange)
         configureMediaControls();
 
     return nullptr;
 }
@@ skipping 239 matching lines @@
 
 #if ENABLE(WEB_AUDIO)
 void HTMLMediaElement::clearWeakMembers(Visitor* visitor)
 {
     if (!visitor->isAlive(m_audioSourceNode) && audioSourceProvider())
         audioSourceProvider()->setClient(nullptr);
 }
 #endif
 
 }