Created attachment 127396 [details]
Screenshot showing the issue

SUMMARY

In X11 with compositing enabled, and in Wayland, the 'Scale' desktop effect results in a transition from the moment a window closes.

There seem to be circumstances where the application can still receive events after the 'Scale' animation has started. This may cause the window to stay permanently drawn over other windows or over the desktop.

Configuring the 'Scale' effect so that it lasts longer does not seem to make the issue easier to reproduce.

I have experienced this with the mpv media player. I could not reproduce the issue in an X11 sesssion.

STEPS TO REPRODUCE
1. Open a file with the mpv media player in a Wayland session.
2. Seek near the end of the media.
3. Click again on the timeline right after the media finished.

OBSERVED RESULT

If you click at the right time, the window is redrawn during the 'Scale' animation and stays there permanently. The whole window can be seen, including the server side decorations. When switching desktops, the drawing stays in the desktop it originally belonged to.

EXPECTED RESULT

Whether the application may receive or react to events during the 'Scale' animation or not (if that's even possible; maybe the application is already dead and it's only kwin causing the issue), a still, translucent drawing of a closed should not stay permanently on the desktop.

SOFTWARE/OS VERSIONS

Operating System: Arch Linux 
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.68.0
Qt Version: 5.14.2
Kernel Version: 5.6.2-arch1-2
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-6200U CPU @ 2.30GHz
Memory: 11,6 GiB of RAM

mpv v0.32.0

glxinfo:
> name of display: :0
> display: :0  screen: 0
> direct rendering: Yes
> server glx vendor string: SGI
> server glx version string: 1.4
> client glx vendor string: Mesa Project and SGI
> client glx version string: 1.4
> GLX version: 1.4
> Extended renderer info (GLX_MESA_query_renderer):
>     Vendor: Intel (0x8086)
>     Device: Mesa Intel(R) HD Graphics 520 (SKL GT2) (0x1916)
>     Version: 20.0.4
>     Accelerated: yes
>     Video memory: 3072MB
>     Unified memory: yes
>     Preferred profile: core (0x1)
>     Max core profile version: 4.6
>     Max compat profile version: 4.6
>     Max GLES1 profile version: 1.1
>     Max GLES[23] profile version: 3.2
> OpenGL vendor string: Intel
> OpenGL renderer string: Mesa Intel(R) HD Graphics 520 (SKL GT2)
> OpenGL core profile version string: 4.6 (Core Profile) Mesa 20.0.4
> OpenGL core profile shading language version string: 4.60
> OpenGL core profile context flags: (none)
> OpenGL core profile profile mask: core profile
> OpenGL version string: 4.6 (Compatibility Profile) Mesa 20.0.4
> OpenGL shading language version string: 4.60
> OpenGL context flags: (none)
> OpenGL profile mask: compatibility profile
> OpenGL ES profile version string: OpenGL ES 3.2 Mesa 20.0.4
> OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.20