Here is a stable way to get the warning. These warnings are expected, because of limitations of X11. But you can use this trick to see if you compiled the correct version of Krita.

1. Right-click on a window title bar and do not release the mouse button
2. Drag the cursor to the canvas without releasing the button
3. Without releasing the button click on the left button to close the popup menu
4. Now drag over the canvas with the button still pressed and you should see the warnings.

If you see these warnings, but you don't see the warnings after remapping the stylus buttons, then it means the code works correctly.

I tried that (what's in the first post) in your branch, and didn't see any related warning. I'll check your next comment..

Tested your next comment, I did see those warnings (except I had to release the right-click button to can left-click on canvas, but it did produce those warnings):

But, after remapping the stylus buttons to default, I can still get the same warnings with right-click on title bar then left-click on canvas (if that's what you meant in "if you see these warnings, but you don't see the warnings after remapping the stylus buttons, then it means the code works correctly").

If you don't see these warnings while usual painting/panning/dragging with the buttons remapped, it means the code works fine. The right-click trick will always work, regardless the button remapping :)