[Reg 5.13.0->5.13.1] mouse position gets confused when second monitor exists

System setup:

• Two displays with mixed resolution (1920x1200, 3840x2160)
• Window Manager: KWin
• 1920x1200 display is to the left of the other one

Observed behaviour:

• When resizing window panes, or dragging elements in the outline, there appears to be a negative x offset (pane immediately resizes to the minimal width, elements have to be dragged to the right in the outline to be moved)

Expected behaviour:

• There is no mouse offset