How to stretch fullscreen a Window

[CrezyDud]

I have a window, rendering at 1440p but want to stretch it to 4k (gpu too slow for 4k)
but how to i stretch the window output fullscreen, because if i rescale/fullscreen normal, i'll just extend the window and have black border on bottom right, how do you do this?

is it even already possible? this works on wayfire (wayfire plugins extra), so if you want to implement, you can help yourself with looking at it's code!

[CrezyDud]

works now, for some reason, but fine!

[CrezyDud]

oh it works because i set it to fullscreen, now this wont work for all things, so i'll leave unasnwered