How to fix shift in y-direction of mouse position

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Hi,

I draw a polygon using mouse clicks and realised some shifts in the returned y-position. 1. when clicking on the top part of the frame, the mouse click position is shifted by a few pixels up 2. when clicking in the lower part of the frame, the mouse click position is shifted by a few pixels down 3. when clicking in the middle of the frame, the mouse click position is correct

I could observe this in my code and could replicate it with the code of the tutorial Draw circles: https://pysource.com/2018/03/27/mouse...

I am using OpenCV 3.4.2, Python-binding, OSX 10.14.3

Thanks for any help

Best Andreas