Youtube theater mode not using full screen width

[struq]

Steps to reproduce

1. Open any wide format video on Youtube, e.g. https://www.youtube.com/watch?v=6Y8FVIiWyao
2. Click the "theater mode" button on the player toolbar (third button from the right)

Expected behavior

1. The video always uses the full screen width, like in Safari and Chrome

Actual behavior

1. Initially the video is not using the full width and there are black bars on the left and right.
2. Only after scrolling down the page, the video is enlarged to the full width (and the resize process is not smooth)

Device & build information

• Device: iPad Pro 10.5"
• Build version: 109.0 (25841)
  1. Notes
     Attachments:
     UNITO-UNDERSCORE!firefox!
     UNITO-UNDERSCORE!safari!

┆Issue is synchronized with this Jira Task

[dholbert]

This is a version of #11574 -- it looks like Firefox is dynamically resizing its viewport depending on the presence/absence of the dynamic toolbar. You can see this by visiting https://www.whatismybrowser.com/ and comparing the "Browser Window Size" between Firefox vs. Chrome/Safari (on iOS). For me on my iPad, Firefox reports 748px vs. Safari/Chrome report 768px (when I'm holding my tablet in landscape mode).

So the root issue here is that Firefox is using a shorter viewport here than Chrome and Safari (and then YouTube sees that the viewport is shorter and it makes the video a bit smaller, because they have some logic to ensure that the video plus a bit of the content that follows it is visible in the viewport-height-that-the-browser-reports-to-them).

[hiikezoe]

Given that the iOS version of Chrome can handle vh units properly, there should be APIs to tell the dynamic toolbar initial height to WebKit and tell the intermediate height to WebKit as well.

[lmarceau]

Hello! Thanks for your input, I believe this relates to #11574 then.

[github-actions]

This issue has been automatically marked as stale. Has the issue been fixed, or does it still require the community's attention? Please leave any comment to keep this issue opened. It will be closed automatically if no further update occurs in the next 30 days. Thank you for your contributions!

[dholbert]

Still reproducible as-described in initial comment. (I just retested with current Firefox release vs. Safari, on my iPad.)