Build the attached reproducer against 6.8.3 or 6.9.0, and run with -platform wayland or -platform xcb to see the issues, respectively. Drag slider around to resize window and glitches should happen.

On Wayland:
Window "wobbles around" (or say "shakes") when being resized.
On XCB:
The Rectangle that fills the window cannot "keep up" with the resizing, leaving right and bottom edges black for a moment.

So far, I'd say it is about either OpenGL itself or how we use OpenGL. Since rendering with software or Vulkan backend works perfectly fine.

Update: a screen record is uploaded. That was captured by original reporter and the app being shown is built against 5.15.18. Nevertheless, that is not a bug concern - app built against Qt6 shows the same behavior.