Reg:Frameless Window when moved 0 0 shifts few pixel to left

If Frameless QMainWindow is move to 00 having width fit to screen it shifts to left fe pixel.
Works good with Qt5.12.2 broken in 5.12.3
Attached example reproduces it:
Steps:
-start the example, the main window is shown at a default position close to the top left corner.
-Click on the button "no titleBar move" the title bar disappears correctly and the window should be positioned at (0, 0) (covering exactly the width of the screen on a standard HD display).
However, the window is shifted a few pixels to the left. If you click the button again, it is repositioned correctly, and from then on, it switches between the both positions with each further click.

Strane is, if the code which is triggered by the button is called in the constructor (commented out in my example project), the window is positioned correctly and will not jump between two positions when "no titleBar move" is clicked