Affected versions

• 88.0a1 (20210222214056)

Affected platforms

• Windows 10x64
• macOS 11.2
• Ubuntu 20

Steps to reproduce

1. Open Firefox and Customize mode.
2. Add a few items to the toolbar.
3. Click on Restore Default/ Undo buttons.
4. Observe the toolbar elements.

Expected result

• Elements from the toolbar are placed accordingly when using the Restore Default/ Undo buttons.

Actual result

• Items from the toolbar are sometimes shifting when using the Restore Default/ Undo buttons.

Regression range

• This was introduced with the new toolbar. Reproducible starting Nightly 87.0a1(2021-02-18).

Notes

• Attached a screen recording: link.
• The shifting issue is intermittent.
• [mstriemer] Entering Customize mode changes the padding on the first toolbar button, it looks like this might be the root cause

You can see this as well if you check the position of the first toolbar button (likely the back button) and then enter customize mode. This is likely the cause of the flickering shown in the video.

https://searchfox.org/mozilla-central/source/browser/themes/shared/browser.inc.css#135-136

I don't have access to the screencast, but looks like the above needs a selector for customize mode toolbarpaletteitem too.

(In reply to Tim Nguyen :ntim from comment #2)

https://searchfox.org/mozilla-central/source/browser/themes/shared/browser.inc.css#135-136

I don't have access to the screencast, but looks like the above needs a selector for customize mode toolbarpaletteitem too.

Added attachment to Bugzilla. Sorry for that.

• Applies it on toolbarpaletteitem for customize mode.
• Stops applying it on every child of the first toolbaritem child, since the content of toolbaritem is unpredictable (zoom-controls, reload/stop, profiler icon & dropmarker.

https://hg.mozilla.org/mozilla-central/rev/709b1dcf5dd6

Verified this issue on Firefox 88.0a1 (2021-03-16) and it is fixed. Tests were performed on Windows 10, macOS 10.15.7 and Ubuntu 20.04.