Created attachment 139367 [details]
video demonstration

SUMMARY
"Dialog Parent" effect in 5.22.1 causes parent window to flash after close of its dialog. It happens with every window and only with dialogs, i.e. "Dim Active" effect works as intended. Video attached. It also happens with any render backend and vsync settings.

STEPS TO REPRODUCE
1. Turn "Dialog Parent" desektop effect on.
2. Switch to window and open any dialog. Dolphin with "New Folder" dialog for example.
3. Observe flash after dialog close.

OBSERVED RESULT
After "fade in" animation window flashes.

EXPECTED RESULT
"fade in" animation finishes without flashes as it was in 5.21.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.22.1
KDE Frameworks Version: 5.83.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: Intel
OpenGL renderer string: Mesa Intel(R) Xe Graphics (TGL GT2)
OpenGL version string: 4.6 (Core Profile) Mesa 21.0.1
OpenGL platform interface: EGL
OpenGL shading language version string: 4.60
Driver: Intel
GPU class: Tiger Lake
OpenGL version: 4.6
GLSL version: 4.60
Mesa version: 21.0.1
X server version: 1.20.11
Linux kernel version: 5.11
Direct rendering: Requires strict binding: no
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used

Loaded Effects:
---------------
kwin4_effect_sessionquit
slidingpopups
kwin4_effect_login
kwin4_effect_logout
screenshot
kwin4_effect_translucency
coverswitch
colorpicker
kwin4_effect_frozenapp
kwin4_effect_dialogparent
highlightwindow
contrast
startupfeedback
kscreen