Incorrect view hides the correct view when using --transform
Background/motivation: I need to do some keystone correction to my computer that is connected to a video projector.
I'm just trying to figure out how to use
--transform exactly, but anyway there is something that's not working quite correctly. I took the example from the man pages (just changed the output):
xrandr --fb 1024x768 --output HDMI-1 --transform 1.24,0.16,-124,0,1.24,0,0,0.000316,1
What happens: The correction first looks like it's correct, but one weird thing I can see is that the transformation isn't linear/affine but rather piecewise-linear (the left edge isn't a straight line but a piecewise line). But when I move my mouse, things get redrawn behind the cursor and what is revealed is the apparently the correct transformation. So, I first see something but when I move the mouse, the view gets redrawn as the cursor moves and what is revealed "behind" is slightly different than what was originally shown. Unfortunately, it's not sufficient to just once hover the whole screen with the cursor, because when I open a menu or resize a window or almost do anything that causes something to be drawn on the screen, the screen gets drawn incorrectly again as in the first place. Again, I can hover with the mouse to redraw parts correctly..
So, it seems as if there are two views: the correct one that gets revealed behind the cursor and the wrong one that shows up by default and whenever something happens on the screen.
I tested on KDE (with LightDM) and i3 (with SDDM) and the exact same issue appears on both computers.
What could be the reason for this? Is the problem in xrandr, display manager, window manager or something else? Any ideas?