WheelEvent on inactive window sometimes gives wrong sign of delta

See the attached test. To reproduce the problem:

1. Launch the test
2. Make sure the window is active
3. Scroll mouse wheel up while hovering cursor over the window, see "++++..." the console output
4. Scroll down, see "----..." in output
5. Make the window inactive, e.g. by activating another window
6. Repeat steps 3 and 4, seeing inconsistent output: in one direction of scrolling both "+++..." and "---" are printed.