[FFmpeg-trac] #8684(avformat:new): WebVTT decoder breaks when ::cue elements are used

[FFmpeg]

#8684: WebVTT decoder breaks when ::cue elements are used
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             Reporter:  derrod      |                    Owner:  (none)
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  avformat
              Version:  git-master  |               Resolution:
             Keywords:  webvtt      |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
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Comment (by logv):

 Note to add that this issue currently seems to break processing of all
 recently produced VTT subtitles served by SVT Play
 (<https://play.svt.se>). I'm not sure if this is perhaps considered
 malformed, or if it is something ffmpeg optimally should be able to
 handle. When trying to convert subtitle format to SRT or ASS, ffmpeg fails
 silently and produces a 0 byte file (SRT) or just boilerplate (ASS).

 E.g., a typical start of a VTT from SVT Play:
 {{{
 WEBVTT

 STYLE
 ::cue {
     color: #DFDFDF;
 }
 ::cue(.huvudpratare) {
     color: #00FFFF;
 }
 ::cue(.ljud) {
     color: #FF00FF;
 }
 ::cue(.ljudskylt) {
     color: #FF00FF;
 }
 ::cue(.relivemark) {
     color: #FF0000;
 }
 ::cue(.sang) {
     color: #00FF00;
 }

 5da33626-c8a3-48d2-b40a-d5878ea108fa
 00:01:34.320 --> 00:01:39.160 align:left position:18%
 <c.teletext>Han kommer senare.
 Vi kan inte göra nånting åt det nu.</c>
 }}}
 Another (world accessible) example lives here (for a limited time):
 https://ed4.cdn.svt.se/d0/world/20220811/b9e3c11c-6cdd-
 46b7-8947-228492d686a5/text/text-0.vtt (also attached)
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/8684#comment:6>
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