Created attachment 151783 [details]
Visual description of the bug

I'm using the transparency effect when moving windows, which was recently disabled by default in KDE Plasma. I recently noticed that when using it, a bug occurs due to which the program window (any window, be it the main, modal or dialog) does not completely disappear when closed. Instead of completely closing the window, it leaves a transparent plume

STEPS TO REPRODUCE
1. Enable transparency effect in System Settings: Workspace Behavior > Desktop Effects, mark the effect "Translucency"
2. Launch any program so that its window appears
3. Grab the window with the mouse cursor to make the window transparent, then close it with Alt + F4

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.25.4
KDE Frameworks Version: 5.97
Qt Version: 5.15.5

ADDITIONAL INFORMATION
With the transparency effect turned off, this bug does not appear. X11 session in use. Fixable only by relogin or rebooting. This bug also works with windows that can close on their own (like progress bars, the Steam login window, etc)