Consider cherry-picking 1cf0520454f8f55256b7f6cdfeefa8360a5842c7

Description:

Firefox (and other GTK3 apps) get wrong screen information in presence of fractional scaling. Sway and Kwin are at least affected, haven't checked Mutter because it doesn't enable fractional scaling by default, but I suspect it's affected as well.

Additional info:

• package version(s): gtk3 1:3.24.43-3
• config and/or log files: N/A
• link to upstream bug report, if any: https://gitlab.gnome.org/GNOME/gtk/-/commit/1cf0520454f8f55256b7f6cdfeefa8360a5842c7

Steps to reproduce:

On KDE Plasma Wayland, make sure to have fractional scaling (e.g., 1.5x), open Firefox terminal and run screen.width.

In my configuration the right answer should be 2560 but is 1920. This causes menus to be cropped too soon for example.