Flickering/Blinking IMSC Subtitles

[mattjuggins]

Flickering/blinking of subtitles happens irregularly during the playback of imsc subtitles. This is clear to witness when playing back a stream that has subtitles that appear word by word (how live subtitles might appear). For example, using the “[BBC] On-demand Elephant's Dream - with EBU-TT-D 'Snaking' Subtitle Track (random text) - lines grow over time simulating live subtitles” stream in the reference player it’s clear to see the subtitles flickering. The majority of cues in this subtitles track appear immediately after the previous cue and hence should appear smoothly appended.

This flickering effect is happening because appending divs and removing divs (that represent cues) from the captionsContainer don’t necessarily happen close enough together to ensure the captionsContainer will not be rendered as empty in between. At present, the appending will happen in _renderCaption() and the removal will happen in the previous cue’s onexit function.

When both these actions are moved within _renderCaption() no flickering is observed.
However, if there is no immediately following cue, the cue onexit event should be preserved.
Additionally, identical cues should still be extended rather than redrawn, and WebVTT subs and CEA608 should still operate as expected.
I have proposed a fix for this in #4359.

[dsilhavy]

This was fixed in #4359