issue/4888-ccOffset

[littlespex]

Ensure the first continuity segment is processed in webvtt-parser's oncue handler.

The first continuity segment is hard coded with new: false. This prevents webvtt-parser from correctly calculating the ccOffset when you seek back to the first continuity segment after starting in a later segment:

webvtt-parser

  parser.oncue = function (cue: VTTCue) {
    // Adjust cue timing; clamp cues to start no earlier than - and drop cues that don't end after - 0 on timeline.
    const currCC = vttCCs[cc];
    let cueOffset = vttCCs.ccOffset;


    // Calculate subtitle PTS offset
    const webVttMpegTsMapOffset = (timestampMapMPEGTS - initPTS90Hz) / 90000;

    /////////////////////////////////////////////////////
    // Only "new" CCs get their cue offsets calculated.
    // This makes sense if you start playback from 0,
    // but causes a problem when `startPosition` is set
    // to a later continuity segment. When you seek back
    // the `cueOffset` remains at the later playhead time
    // and new cues are added at the wrong time.
    /////////////////////////////////////////////////////

    // Update offsets for new discontinuities
    if (currCC?.new) {
      if (timestampMapLOCAL !== undefined) {
        // When local time is provided, offset = discontinuity start time - local time
        cueOffset = vttCCs.ccOffset = currCC.start;
      } else {
        calculateOffset(vttCCs, cc, webVttMpegTsMapOffset);
      }
    }

Resolves issues:

#4888

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md