Found in

• Nightly 108.0a1

Affected versions

• Nightly 108.0a1

Tested platforms

• Affected platforms: Ubuntu 20.04, macOS 11.7, Windows 10

Steps to reproduce

1. Reach YouTube and start playing a video
2. Click on the PiP button.
3. Mouse hover the buttons of the PiP window.

Expected result

• The PiP window should be properly displayed while the mouse hovers the buttons.

Actual result

• Scrollbars are displayed after the mouse hovers the buttons. Please see the screen recording for more details.

Regression range

• First bad revision: 73dd9b390b9a0712ea44b2345966c54ebc83c492
• Last good revision: 8108291015f4419be14591ead2d59d6235e7e601
• Pushlog:
  https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=8108291015f4419be14591ead2d59d6235e7e601&tochange=73dd9b390b9a0712ea44b2345966c54ebc83c492
• Potentially regressed by: Bug 1799925 - Make XUL popups contain: paint. r=dholbert

:emilio, since you are the author of the regressor, bug 1799925, could you take a look?

For more information, please visit auto_nag documentation.

So that they don't affect the overflow area of the root element. Since
they paint in a different widget over the current one it doesn't make a
lot of sense for them to do so.

Backed out for crashes on FragmentOrElement.cpp

Backout link: https://hg.mozilla.org/integration/autoland/rev/6479051196c1165c23a1964a00422e3be55f7ff1
Log link: https://treeherder.mozilla.org/logviewer?job_id=396362271&repo=autoland&lineNumber=3449

Please take a look at these leaks:

1. https://treeherder.mozilla.org/logviewer?job_id=396363245&repo=autoland&lineNumber=5189
2. https://treeherder.mozilla.org/logviewer?job_id=396363235&repo=autoland&lineNumber=6061

https://hg.mozilla.org/mozilla-central/rev/adae4eadc28d

Reproduced with Nightly 108.0a1 from 11.11.2022 on Windows 10, Ubuntu 20 and Mac 12 ARM.

Issue is verified fixed in latest nightly and in 108.0b3 Firefox on all OS.