To lower memory used  (ROM), I'm using scaled bitmap for my background.

For some reasons, the x2 is not exactly x2 and so,  when you try to stack objects vertically, you sometimes skip 1 or 2 lines

just WHY ?!
Why the first interesting feature I use is just bugged.... ?

it seems to be known (emulators handle it perfectly) but I didn't find the explanation/reason yet...

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Move to something easier : set the background color !

Well..no, try again :facepalm;

the BG should be set with a CRY value (the Jaguar color space...ironically created for gouraud shading)

I tried EVERY tool available and it seems impossible to ask for a blue royal...