The root of it, I would guess - and this is purely gut feel: I haven't looked at the code - is the obsession with smooth scrolling and/or "momentum" scrolling that's appeared since 16.04. Experimenting with 19.04 today, after staying on 16.04 all this time, is just endlessly frustrating, because almost NOTHING works properly or well. But the scrolling issues are WAY up there, along with the filechooser amnesia.
I take it it's too much to hope that there's a #define lurking somewhere in the source to just turn it off and have 1 atom of input translate to one atom of movement again, instead of 0 half the time and 2d6 the other half?
The other thing I wanted to mention is this: https://aur.archlinux.org/packages/gtk3-mushrooms/
I expect you've long since seen it, but I only found out about it today and figured it was worth bringing up just in case. He admits he basically doesn't know how to code, but it might ease some of the burden occasionally?