Automatic and Continuous MIDI Editor Scrolling in REAPER 

This video hopefully helps anybody looking to solve the elusive continuous scroll for the MIDI Editor. Continuous scrolling in the MIDI editor is achieved by syncing with the scroll function of the Arrange view.
Follow these steps: Arrange view (main track editing window), OPTIONS Continuous scrolling. MIDI Editor view Custom Actions, Timebase: Toggle sync to arrange view.
Jon TIdy's Reaper Blog / @thereaperblog
Kenny Gioia's Reaper Mania / @reaper_mania