For those who used Ubuntu in the days of GTK2, you might know GTK3 introduced some quirks (and regressions) of much liked aesthetics and functionality. A project named gtk3-mushrooms patches up GTK3 to restore them like old times.
It is only packaged up for Arch (AUR) and Manjaro (as
gtk3-classic), so it's not available for those using Ubuntu MATE. It should be noted that Ubuntu also apply their own patches to Ubuntu's GTK3 which may or may not cause other issues. These patches do cause a conflict so
quilt is necessary to unpatch some of all of Ubuntu/Debian's patches.
I spent hours already trying two approaches to get it working on Ubuntu, without 100% success. So, I left my notes if anyone is interested in having an attempt to compile/package this project. GTK3 is a large beast, so it will require patience.
Packaging would be better then compiling/installing alongside the system's GTK3. Plus, it can easily be used in a PPA and built by Launchpad. I ran into problems just compiling
gtk+3.0 (19.04) packages even without applying any additional patches.