Just a head's up, I've been working on this myself actually. There are some big changes involving the mouse code (and maybe some others) that I may not have merged properly so I'll ask you to take a careful look at the merged features when I have it ready, since you know the code better than myself, but I've made quite a bit of headway here... a lot of the conflicts are entirely trivial.
Since I knew about this only now, I would like to ask to create a separate branch for this (forking from current develop-3.4.0), and do several commits: first a merge commit with only trivial items merged, and then separate commits each reimplementing a single feature/change (that requires different reimplemention). Then, this should be done as a pull request (I was myself planning to make one) for review and fix if required, etc.
I was mostly wondering about graphics mode init and winsetup code, because they were rewritten completely. Some fixes to gfx init made in 3.3.5 may not need to be carried over at all.
E: Also, idk if you are aware of that, but I noticed that when you were updating 3.3.5 branch in your repository you created redundant "merge" commits instead of fast-forward update; could it be that you are using "always create merge commit" option?