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 https://github.com/mozilla-mobile/firefox-ios/issues/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 https://github.com/mozilla-mobile/firefox-ios/issues/11574 then.

[github-actions[bot]]

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.)

[TechyAditya]

Will this get fixed anytime soon? The issue is still there even after 2 years

[data-sync-user]

➤ Yoana Rios Diaz commented:

Fixed by the work done for https://mozilla-hub.atlassian.net/browse/FXIOS-11527 ( https://mozilla-hub.atlassian.net/browse/FXIOS-11527|smart-link ) that enables fullscreen

[data-sync-user]

➤ Andrei Bodea commented:

Hello, Yoana Rios Diazplease note that I’m still able to reproduce this issue please check the video. Can you please explain how this fix should work, or how it can be verified?

[^ScreenRecording_04-09-2025 15-11-04_1.MP4]

[data-sync-user]

➤ Yoana Rios Diaz commented:

Hi Andrei Bodea. I’ve been investigating which will be the best way to test this ticket but I’m not sure. I don’t think it should be based only if there is vertical or horizontal black space around the video. This is Firefox 137 without fullscreen

!firefox-137-without-fullscreen.PNG|width=2048,height=1536,alt="firefox-137-without-fullscreen.PNG"!

Firefox 138 with full screen

!Firefox-138-with-fullscreen.PNG|width=2048,height=1536,alt="Firefox-138-with-fullscreen.PNG"!

And Safari as reference

!Safari.PNG|width=2048,height=1536,alt="Safari.PNG"!

As you can see there is horizontal black space in the three cases. One difference is that for Safari and Firefox 138 (with fullscreen) we get a toast like message informing we are entering full screen (see image below)

!Simulator Screenshot - iPad Air 13-inch (M2) - 2025-04-09 at 13.42.36.png|width=2048,height=2732,alt="Simulator Screenshot - iPad Air 13-inch (M2) - 2025-04-09 at 13.42.36.png"!

In this image the device was rotated so you can see how the space changes I this is based on the screen aspect ratio We assumed that supporting Full screen would fix this issue (according to acceptance criteria) but I’m not sure if this is behaving as expected or if we need to do something else. At least it is matching Safari behaviour

[janbrasna]

I'm struggling to see the "correct" dimensions reported. https://www.whatismybrowser.com says "BROWSER WINDOW SIZE Including your tool bars" which I'd take should not change on scroll, as the size with toolbars equals the content when no toolbars are shown, but both FxiOS and CriOS show a range between the extremes.

On a physical iPad mini5 iPadOS 26.1, FxiOS is shown starting at 608px and ending at 698px:

whereas Chrome iOS starts at 587px and changes while scrolling all the way to 667px:

which I'd expect to be constant really from the description… or is the site just naming things incorrectly (or oversimplified)?

But that would seem to actually provide a taller viewport on Firefox than in Chrome, IIUC?