https://bugs.kde.org/show_bug.cgi?id=489952

--- Comment #35 from S. Christian Collins <[email protected]> ---
Created attachment 184082
  --> https://bugs.kde.org/attachment.cgi?id=184082&action=edit
image illustrating uneven cursor trail

I have experimented with setting the KWIN_DRM_OVERRIDE_SAFETY_MARGIN
environment variable between 2000 and 5000, and here is what I've found on my
144 Hz display:
* 2000: maybe slightly better
* 3000: improved timing, but not yet perfect
* 4000: still not perfect
* 5000: almost perfect, but still some frames out of place

With KWIN_DRM_OVERRIDE_SAFETY_MARGIN=5000, I was able to get a better glimpse
at what is actually happening. It's not that cursor frames are being dropped,
but rather the mouse movement is occasionally being "snapshotted" too early or
too late. The attached image illustrates what I am seeing when moving the
cursor horizontally for 8 frames. In this example, all 8 frames are present,
but not all of them are spaced equidistantly as they should be.

Lastly, I have discovered another scenario where the cursor moves perfectly
smoothly:
1. Right click on a program in the task manager (I am using the Icons-and-Text
Task Manager, if that matters).
2. Move the cursor within the region of the menu that pops up.

Result: the cursor movement is perfectly smooth.

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