What is your native resolution? I use awesome window manager, so if I hide the top bar, the only indication that anything’s changed is that the wallpaper looks horrendous. But given that I reset it right back after darktable ends, as long as I don’t switch applications, I don’t think anything should be too noticeable? I think anything that’s DPI-aware shouldn’t have too many problems (although you would have to reset e.g. GDK_SCALE if you want to start a new program while darktable is running).
Huh, that is weird. I wouldn’t really know since I use a tiling window manager (so I don’t normally resize clients manually).