The patch in bug 738335 introduces a 1px white flickering line at the top of the window in fullscreen. See https://phabricator.services.mozilla.com/D106847 for the current debugging efforts. We should land the patches current in that bug so they don't become bitrotted and so QA can test the behaviour, but we need to address the flickering line before enabling them. With I'll land the patches in this bug since bug 738335 won't be complete until the behaviour is enabled by default.

Depends on D106847

(In reply to Harry Twyford [:harry] from comment #0)

The patch in bug 738335 introduces a 1px white flickering line at the top of the window in fullscreen. See https://phabricator.services.mozilla.com/D106847 for the current debugging efforts. We should land the patches current in that bug so they don't become bitrotted and so QA can test the behaviour, but we need to address the flickering line before enabling them. With I'll land the patches in this bug since bug 738335 won't be complete until the behaviour is enabled by default.

Hi Harry!
Can you please help us with some steps on how we should verify this fix after it lands? Or some scenarios to test? Thank you!

Please see the comment above! Sorry for spam

Hey Alex!
I added a screen cast that shows the difference between the old behavior and the new behavior.

In the current version, when Firefox is in fullscreen and you move the cursor to the top of the screen, the menu bar will slide on top of the Firefox UI. With the patch applied, the Firefox UI will slide down.

So in order to verify this, move Firefox to fullscreen with the pref flipped and try to trigger the menubar by moving the cursor to the top of the screen :)

https://hg.mozilla.org/mozilla-central/rev/f4dfcad8b881
https://hg.mozilla.org/mozilla-central/rev/8ec37432c5b1
https://hg.mozilla.org/mozilla-central/rev/507074c89a49

(In reply to Martin Balfanz [:mbalfanz] from comment #7)

Created attachment 9210225 [details]
side-by-side.2021-03-17 09_25_58.gif

Hey Alex!
I added a screen cast that shows the difference between the old behavior and the new behavior.

In the current version, when Firefox is in fullscreen and you move the cursor to the top of the screen, the menu bar will slide on top of the Firefox UI. With the patch applied, the Firefox UI will slide down.

So in order to verify this, move Firefox to fullscreen with the pref flipped and try to trigger the menubar by moving the cursor to the top of the screen :)

Thank you, Martin!
We will verify this after landing in Nightly using full-screen-api.macos.shiftToolbar and native fullscreen pref and report here. Clearing the ni? request.

Just to be sure, this is the same line that comment 0 talks about? So basically it is a known issue? Thank you!

Yes, that's how I understand it. I can't reproduce the white line myself on 10.15 with an external display.

(In reply to Martin Balfanz [:mbalfanz] from comment #11)

Yes, that's how I understand it. I can't reproduce the white line myself on 10.15 with an external display.

My colleague can't either on macOS 10.15 so maybe this is macOS 11.2.3 related. Waiting for the confirmation from Harry as well regarding the white line from comment 10.

Yes, that's the white line. Interesting that it can't be reproduced on 10.15. I can reproduce it on macOS 11.1 on both my MacBook display and an external display. Alexandru, if you team finds that the line can't be reproduced on macOS <11, we might consider preffing it on for those users.

In any case, knowing whether it's only a macOS 11+ issue will help with debugging.

Confirming that we cannot reproduce the issue on macOS 10.12, 10.13, 10.14, and 10.15. But on those, we can reproduce bug 1699688. So basically macOS 11 has a smooth transition and white line and macOS <11 has a jumpy transition and no white line.