Firefox window loses maximized state after leaving fullscreen video

Windows Version Number: Windows 11 26H2
App Version: 2.31

Found a bug with Firefox on Groupy

Steps:
1. Have a maximized group with a few Firefox windows in it
2. Open a new Firefox window that is NOT maximized
3. Drag it into the group (Groupy maximizes it to fit, all good so far)
4. Play any video in that tab and go fullscreen
5. Exit fullscreen

The window comes back at its old non-maximized size instead of maximized, and then every other tab in the group gets resized to that smaller size too. I have to maximize it again by hand and the whole group follows.

It only seems to happen with windows that were not maximized before they joined the group. Windows I maximized myself first are fine.

My guess is Firefox never notices that Groupy maximized it. Firefox remembers the window state from before fullscreen and puts it back on exit, and from what I can tell it only updates that state when it gets a window pos change with SWP_FRAMECHANGED. So it still thinks the window is "normal" and restores it.

Demo video

https://commons.wikimedia.org/wiki/File:Big_Buck_Bunny_4K.webm