Hot on the heels of the big 4.5 update, only a few days later 4.51 was released with a few more MIDI updates.
Changelog:
+ MIDI editor: dropdown box above keyboard area to select active track
+ MIDI editor: better performance updating edits with lots of pooled items
+ MIDI editor: improved source length editing behavior
+ MIDI: faster creation of pooled midi items
+ MIDI: fixed stuck note on looped playback introduced in 4.5
+ Undo: reduced excess work/flicker when in an undo block