Clients mapped in fullscreen state get additional WGroupCW when switched to windowed state

[C4tion]

Steps to reproduce:

1. Create a window that is fullscreen from the start: mpv bbb_sunflower_1080p_60fps_normal.mp4 -fs
2. Toggle fullscreen with Notion keybinds.

Observed behavior:
The title of the windowed mpv is now WGroupCW<...>.

Expected behavior:
The title should be bbb_sunflower_1080p_60fps_normal.mp4 - mpv.

This might be related to #247, but the issue described there is not fixed by my patch.