Every time I open Gnumeric the window extends beyond the physical screen and is cumbersome to resize/reposition every time.
Through some web searches I found out that there is an option in KWin to permanently set startup window settings for a given application.
However, it seems I'm missing the tabs that actually do the modifcations. Instead, I only have 1 tab that has the rules for how to detect the window.
I could not find any way to actually set any rules.
I'm using kwin 5.20.4 on Arch.

Click on "Add Properties" in the lower left of the dialog...