Subtitles requiring .attachTTMLRenderingDiv(TTMLRenderingDiv) fail in Firefox

[wilaw]

The sample player at samples/captioning/ttml-ebutt-sample.html will fail to show the captions in Firefox 44. We suspect that the layering mechanism within dash.js is at fault when positioning this layer above the video.

[davemevans]

I think this is a duplicate of one of the issues in #748.

This is happening because Firefox does not dispatch enter and exit events which are currently required to display the captions - see https://bugzilla.mozilla.org/show_bug.cgi?id=996333, and also https://bugzilla.mozilla.org/show_bug.cgi?id=1131952.

A workaround might have been to use the cuechange event on TextTrack, but support for that in Firefox is still being implemented - https://bugzilla.mozilla.org/show_bug.cgi?id=1033144

[davemevans]

This seems to have been closed by a PR which fixed an unrelated problem. I think it's still an issue.

[TobbeEdgeware]

@bbcrddave I agree that the problem is still there with Firefox, but the fix was not unrelated since HTML rendering was removed for all browsers due to Firefox problems.

[davemevans]

TextTrackCue onenter and onexit have recently become available in Firefox Nightly - see https://bugzilla.mozilla.org/show_bug.cgi?id=882718. This will become GA in November (assuming it is not backported to an earlier version) so we will be able to enable HTML captions then.

TextTrack cuechange mentioned above is now available in the release channel (v47).

[davemevans]

The required Firefox features are in v49 which ships in mid-September (https://wiki.mozilla.org/RapidRelease/Calendar).