Weird - I could have sworn they were in this thread.
The mousewheel just randomly doesn't work at times, notably in caja windows (which I tend to scroll through that way a lot). Just now, I went through 4 clicks on the wheel in there and absolutely nothing happened at all. Then it suddenly "woke up" and started working.
If I switch to a different window (click to focus) but then hover over the caja window and scroll the wheel, it will almost always ignore the first event but then respond to subsequent ones. Give the caja window focus then switch away from it again and start over, and the first wheel movement gets ignored again.
I never filed a bug for it, for several reasons: because I couldn't imagine it not being fixed by the time I would have first considered actually switching to 18.04 (i.e. several months after release) ; because I've seen how the GNOME team handles bug reports, so it would have been a waste of time if GTK3 was the cause (which seemed likely) ; and because I didn't have the time to pin down exactly when it failed, since it does work properly sometimes - it's only just now that I noticed that it always misses the first event if unfocused, for example. "Sometimes it doesn't work" isn't exactly the sort of bug report that encourages even the most conscientious developers to spend time trying to reproduce and fix things. 
[GTK4] is a very low priority for now.
Thanks - that's what I was hoping to hear. 