Created attachment 187665 [details]
Firefox log file (MOZ_LOG="timestamp,WidgetPopup:5" MOZ_LOG_FILE="popup").

SUMMARY
The position of the drop down menu is in the wrong position, when following the steps on the scaled monitor.

I initially reported this on the Firefox bug tracker (https://bugzilla.mozilla.org/show_bug.cgi?id=2005452), but I was told to report it against KWin instead.

Video showcase: https://bug2005452.bmoattachments.org/attachment.cgi?id=9532634

STEPS TO REPRODUCE
Monitor setup:
- Primary monitor (2560x1440, 100% scaling).
- Secondary monitor (2160x3840, 150% scaling).

1. Open Firefox and move the window to the scaled monitor.
2. Create a folder in Bookmarks Toolbar.
3. Create another folder, within the newly created folder.
4. Populate that folder with a couple bookmarks.
5. Move Firefox to the second monitor. (150% scaling)
6. Open the nested folder.


OBSERVED RESULT
The position of the drop down menu is in the wrong position, when following the steps on the scaled monitor.

EXPECTED RESULT
It should behave like the non-scaled monitor.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora Linux 43
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.21.0
Qt Version: 6.10.1

ADDITIONAL INFORMATION
- Reproducible in Firefox Stable 146.0.
- Reproducible in Firefox Nightly 148.0a1 (2025-12-13).