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This is a known bug and tends to occur when there are rounding issues in the scale (So mostly fullscreen)
There is no fix yet, and you'd have to rewrite a good portion of the rendering just to fix it.
Sorry, I was unclear. This issue tends to occur in fullscreen.
Windowed is fine and from what I see other fan games do is the default medium, or a small size, which are at a x1 and x0.5 ratio respectively.