EGLFS KMS dual screen setup not working ok for Qt Widget

hello,

i put together a small code snippet for QtCreator, to better showcase the issue i see in my dual-head setup with a Qt widget app running through EGLFS on a libMali.so/DRM/KMS setup on Allwinner A20 mainline based OS:

 https://github.com/aventuri/widgetMultiScreen

the outcome is the wrong display of the two (yellow and red) rectangles with the code above only on the ext HDMI diplay. see:

https://drive.google.com/open?id=1tGJxoBzBNn_wEhCi5mnI5a5zLSKIrAwX

the puzzling issue is that if i comment out the w2.show() line:

https://github.com/aventuri/widgetMultiScreen/blob/master/main.cpp#L3

i can see (correctly) a single yellow screen on the primary LCD display.

can someone point me toward better debugging? is a GDB step by step approach the best one?  the Qt stack is that huge..