Mind the gap?

[Worldeater 2]

Does anyone know a trick to get rid of gaps in (theoretically) adjacent controls? These gaps show up with certain settings of "Interface size" and "Interface resolution".

The problem is probably caused by using floats for positioning. Having an additional scale factor applied (the last number returned by getResolution) makes things even worse (more rounding errors). The "window" above looks fine with a resolution of 1440x900 pixels with a scale factor of 0.7 ("Small") but literally falls apart with a scale factor of 0.85 ("Normal").

Overlapping the controls is not an option here (the border elements are buttons and overlapping them would result in "z-fighting": the active one would come to front hiding parts of the "window" or other border elements.

Can anybody think of a solution for this?