Since some time ago I could not scroll in any GTK+ 3 window in StumpWM with an external mouse. Today I found a workaround for this problem: executing
export GDK_CORE_DEVICE_EVENTS=1 in a shell fixes the problem. To set this automatically when starting StumpWM insert the following in your
;; bugfix for scrolling doesn't work with an external mouse in GTK+3 apps.
(setf (getenv "GDK_CORE_DEVICE_EVENTS") "1")
This bug was also reported (and fixed) at the following bug trackers. However, none of those fixes worked for me.