When using multiple displays of different orientation the Application Dashboard gets the offset of the display from the x11-framebuffer wrong. My display starts at +0+560 (xrandr format) but the blurred background is calculated from -560px (vertical) outside of the framebuffer instead if from +560px. The result is some graphics memory from outside the framebuffer leaking, resulting in a flashing background of mostly garbage [as in upper half of attachment]. Everything looks like it should when pressing ALT plus grabbing the Application Dashboard. [as in bottom half of attachment]
Created attachment 110439 [details] Background leaks from outside framebuffer
Created attachment 110440 [details] How it should look (while dragging window)
I just realized I get the exact same problem with the blurred background of the fullscreen "logout menu". Also, I only get this with X11, works correctly with Wayland.
Seems fixed