This is a bit of a refactoring for how we deal with mouse movement and hovering over regions. In order to support mouse hover in File Browser and Outliner we had to add new operators to just for that.
It is a bit weird having such operators bound to "Mouse Move" since they would not work with any other bindings. I also can't imagine having to make new operators for every place where we might want hover.
This patch specifically adds new callbacks for "mouse_enter", "mouse_leave" and "mouse_move" to ARegionType. Together they can be used for hover effects, or setting the mouse cursor based on special processing, etc.
After applying this patch and compiling, go into Preferences / Keymap and delete both operators bound to "Mouse Move":
Afterward the hovering in Outliner and File Browser will continue to work as usual.