Manual stacking of favorited and non-favorited items does not work.
I'd expect them to stack and be favorited afterwards.

This also prevents restocking e.g. favorite arrows with non-favorite arrows from a chest.

(also https://quark.vazkii.us/#management-favorite-items states favoriting is done with alt-clicking, which I'd expect to be alt-leftclick, but it is alt-rightclick. Maybe this ambiguity could be cleared up)

There's really no way to fix this without breaking a bunch of stuff. I'll fix the site though.