fix: Keep TTML subtitles visible until their end time when they extend past a segment

[abolfazl-moeini]

Description

On DASH streams using fragmented MP4 containers for TTML subtitles, cues with an explicit end attribute that extends past the current segment boundary were being clipped at segmentEnd. This caused subtitles to disappear roughly 0.7 s early in the reported case (and similar for other streams).

ExoPlayer / Media3 handles these cues correctly.

Root cause and changes

A previous fix for duplicate cues across segments (#4631) applied:

start = Math.max(start, segmentStart)
end   = Math.min(end,   segmentEnd)

for non-HLS content. While this eliminated duplicates, it also truncated cues that legitimately continue past the segment and are not repeated in the following segment.

This patch:

• Only clips the start time (and only when the cue begins before the current segment — i.e. a continuation).
• Removes the unconditional end clipping for DASH.
• Adds shaka.text.Utils.isCueStrictSubset + logic in UITextDisplayer.append() so that when the full cue and a tail copy both arrive we only keep the longer one and avoid on-screen duplicates (preserving the EBU subtitles regression in 4.1.3 - duplicate line on screen #4631 behaviour).
• Leaves the HLS path untouched.

Testing

• python3 build/check.py — clean.
• python3 build/test.py --quick --filter="trims cues to segment boundaries|isCueStrictSubset" --uncompiled — 8/8 passing on Chrome & Edge (other browser launcher failures are local env only).
• Red/green: the updated expectations fail against the old clipping logic and pass with this patch.

Fixes #10172

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[shaka-bot]

Bundle Size Report for PR #10264

File	HEAD	Base	Diff
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controls.modern.css	30.5 KiB (6.1 KiB)	30.5 KiB (6.1 KiB)	0.0 KiB
demo.compiled.debug.js	265.6 KiB (53.8 KiB)	265.6 KiB (53.8 KiB)	0.0 KiB
demo.compiled.js	265.6 KiB (53.8 KiB)	265.6 KiB (53.8 KiB)	0.0 KiB
demo.css	160.2 KiB (25.7 KiB)	160.2 KiB (25.7 KiB)	0.0 KiB
demo.modern.css	144.1 KiB (24.7 KiB)	144.1 KiB (24.7 KiB)	0.0 KiB
locales.js	44.1 KiB (10.9 KiB)	44.1 KiB (10.9 KiB)	0.0 KiB
receiver.compiled.debug.js	151.1 KiB (24.4 KiB)	151.1 KiB (24.4 KiB)	0.0 KiB
receiver.compiled.js	151.1 KiB (24.4 KiB)	151.1 KiB (24.4 KiB)	0.0 KiB
shaka-player.compiled-es2021.debug.js	1285.7 KiB (318.7 KiB)	1285.4 KiB (318.6 KiB)	+0.3 KiB
shaka-player.compiled-es2021.js	682.0 KiB (228.9 KiB)	681.8 KiB (228.8 KiB)	+0.2 KiB
shaka-player.compiled.debug.js	1568.7 KiB (361.0 KiB)	1568.3 KiB (360.9 KiB)	+0.4 KiB
shaka-player.compiled.js	791.1 KiB (258.6 KiB)	790.8 KiB (258.5 KiB)	+0.3 KiB
shaka-player.dash-es2021.debug.js	955.6 KiB (241.6 KiB)	955.3 KiB (241.5 KiB)	+0.3 KiB
shaka-player.dash-es2021.js	478.5 KiB (163.9 KiB)	478.3 KiB (163.8 KiB)	+0.2 KiB
shaka-player.dash.debug.js	1164.8 KiB (273.2 KiB)	1164.4 KiB (273.1 KiB)	+0.4 KiB
shaka-player.dash.js	557.6 KiB (185.1 KiB)	557.3 KiB (185.0 KiB)	+0.3 KiB
shaka-player.experimental-es2021.debug.js	1679.3 KiB (405.1 KiB)	1679.0 KiB (405.0 KiB)	+0.3 KiB
shaka-player.experimental-es2021.js	939.1 KiB (299.9 KiB)	938.8 KiB (299.9 KiB)	+0.2 KiB
shaka-player.experimental.debug.js	2028.8 KiB (456.9 KiB)	2028.4 KiB (456.9 KiB)	+0.4 KiB
shaka-player.experimental.js	1071.7 KiB (335.6 KiB)	1071.4 KiB (335.5 KiB)	+0.3 KiB
shaka-player.hls-es2021.debug.js	979.1 KiB (245.6 KiB)	978.8 KiB (245.5 KiB)	+0.3 KiB
shaka-player.hls-es2021.js	506.7 KiB (172.9 KiB)	506.5 KiB (172.8 KiB)	+0.2 KiB
shaka-player.hls.debug.js	1192.4 KiB (277.7 KiB)	1191.9 KiB (277.6 KiB)	+0.4 KiB
shaka-player.hls.js	588.6 KiB (195.4 KiB)	588.2 KiB (195.3 KiB)	+0.3 KiB
shaka-player.transmuxer-worker.debug.js	190.2 KiB (46.5 KiB)	190.2 KiB (46.5 KiB)	-0.0 KiB
shaka-player.transmuxer-worker.js	83.5 KiB (28.4 KiB)	83.5 KiB (28.4 KiB)	0.0 KiB
shaka-player.ui-es2021.debug.js	1597.1 KiB (387.6 KiB)	1596.8 KiB (387.5 KiB)	+0.3 KiB
shaka-player.ui-es2021.js	899.0 KiB (288.3 KiB)	898.7 KiB (288.2 KiB)	+0.2 KiB
shaka-player.ui.debug.js	1916.5 KiB (435.4 KiB)	1916.1 KiB (435.3 KiB)	+0.4 KiB
shaka-player.ui.js	1020.0 KiB (321.4 KiB)	1019.7 KiB (321.3 KiB)	+0.3 KiB

[shaka-bot]

Incremental code coverage: 97.37%

[avelad]

The behavior is changing because the layout tests are failing