[TingPing]

The GTK team is like 3 people. GTK4 worked on improved cross-platform support, effecient gpu rendering, documentation, sandbox integration, etc.

UI designers are probably limited and there is just a lot of drama about the filechooser so it's easy to find other things to work on.

[hda2]

The patches already exist. In fact, PopOs! already has this feature merged.

[superkuh]

I searched PopOs! repos and I couldn't find where they maintained their Gtk-3.0 patches or code. It didn't look to me like they altered Gtk-3.0 itself at all and likely just altered individual applications.