FMod - Middle Mouse Drag

A person on the Keen:Modding IRC channel suggested that I add a feature from the "Mindbelt" Vorticons editor: drag the level view around using the middle mouse button. That sounds very useful for people who have a middle mouse button, so I mapped one of my extra pen actions to it for testing purposes. It wasn't terribly difficult to implement, but it definitely would have been easier if scrolling the opposite direction extremely quickly had been acceptable. This change will appear in the next major update.