I had a stab at (re)implementing this as I know the MATE Desktop project had this issue when migrating from GTK2 to GTK3.
https://github.com/lah7/geany/commit/5bfad0da637836b63d087b791e7b26ebcb51258...
This only reinstates scrolling over tabs in the main editor. I'd consider the commit incomplete as all the other tabs (`notebook`?) like the sidebar and preferences should add the scroll event too but I'm terribly unfamiliar with C and didn't have much luck at this point. _Any help greatly appreciated._
But... it's a step towards having it back in GTK3 and doesn't impact the GTK2 build.
--- Scrolling over tabs is definitely part of my workflow. Too bad GTK3 requires each app to implement it separately. Unfortunately for me, as I switched distributions and finally made use of my HiDPI display, GTK2 is out of the question, _as much as I like GTK2._