Think about making the mod interface fullscreen (only)
Does anyone benefit from the mod not being fullscreen? Does it do anything? I'm not sure.
There are a lot of 'hacks' or tricks required to make the current setup work, a lot of which could be dropped if the mod is made fullscreen. The GUI would need significant adjustments to work at any size, instead of the well-defined steps we have now. Should be possible by using the subfactory list width and height as 'fudge' space.
Having the whole screen be UI would also feel kinda bad I feel, losing the connection to the factory. This could be helped by incorporating a transparent section in the UI, or several. The technology GUI does this, and it seems it's possible for mods to do as well. The basic idea is to make the base frame transparent, which gets rid of borders, which will need to be recreated by adding margin/padding to the individual child frames. Should be doable.
Due to the significant changes that this would entail, it's not really feasible to offer both fullscreen and not, it'd make the code too complicated.
Honestly, I think this is the way to go, but I'm not sure I'm up for the effort required.