User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36

Steps to reproduce:

• Load https://elements-demo-vanilla-l933ni94n-mux.vercel.app/vanilla-video-2.html
• Confirm CC is on
• Play/Seek to ~23 seconds
• Confirm no tracks are rendered
• Using dev tools, confirm track.mode = showing
• Using dev tools, confirm active cues/cuechange events are present when cues should render
• Using dev tools, assuming devtools.inspector.showAllAnonymousContent & devtools.inspector.showUserAgentShadowRoots are true, confirm no cues have been added to the "captions-box" anonymous div

Additional Notes:
with the help of Wesley Luten (https://github.com/luwes), it looks like this may be related to the CSS structure on the example page mentioned above. That said, we experienced a number of permutations that would unreliably reproduce the issue. Additionally, in case this helps, for some cases where cues would stop rendering, we could get them to start rendering again by restarting FireFox.

The same test page (and all other permutations where we experienced the issue) reliably render in both Safari and Chrome.

Actual results:

No cues were rendered in spite of the mode being showing and the active cues being updated.

Expected results:

Cues should have rendered.

The Bugbug bot thinks this bug should belong to the 'DevTools::Inspector' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Paul, curious if this is audio/video related or something in the front end. Appears to have something to do with the css or layout of closed captioning on video.

Kind of both. alwu will know what to do about this.