they should have implemented native dialogs when they had the choice!
this is currently limited to firefox and not applied globally because I
am certain it won't be working with everything GTK. so, to prevent random
breakage only firefox for now (which is the single gtk app we ship
subject for evaluation come 20.04; I fear for 18.04 portal tech in general
is too new to expect everything to work properly.
this relies on portal support being fully provided, which is enabled via
neon-desktop. might move somewhere lower in the stack even, it's a pretty
central part of the containment story.