SetRenderTranslation puts widget at twice the position on screen

Hi

I try to put a widget at a position projected from 3D to 2D
here I forced the position to 512x512

pasting the print screen in gimp, I get 1024x1024 (runing fullscreen)
I get ± 800x800 non fullscreen

why is it so cryptic ? is there something I am missing ?

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canvas slot / set position gives me the same thing

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