Created attachment 165440 [details]
Slow-motion screen recording

SUMMARY
When de-maximizing windows by dragging them with the mouse, the window-restore effect is spread across multiple presentation frames: First, the window "unsnaps" from the top of the screen, and it only shortly after that it resizes to its original size.

In effect, it looks as if the window jumped around before restoring its actual size. This is particularly noticeable with the wobbly windows effect, where the window corners will be unevenly stretched at its corners.

I attached a slow-motion video to illustrate: Stepping through it frame by frame you can see that 1. The window unsnaps (note the top-left corner being on a different height than the top-right one), 2. The wobbliness calms down but the window is still in maximum size, 3. The window resize kicks in, triggering another wobble.

This might be a Wayland-specific issue.

STEPS TO REPRODUCE
(0. Test on Wayland)
1. Maximize a window
2. Drag the window from its title bar to restore its size
3. Closely observe the result, as this glitch happens within 2-3 presentation frames

OBSERVED RESULT
The restore operation is non-atomic: The window unsnaps before being resized

EXPECTED RESULT
The window should start resizing in the same frame that its top-left corner moves.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.27.10
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.114.0
Qt Version: 5.15.12

ADDITIONAL INFORMATION
I'm also observing this glitch in a Parallels VM, where it can be prolonged by continuously moving the mouse cursor. This will delay the window resize until the cursor stops moving. Maybe this gives a hint on the nature of the issue?