I spent a long time fiddling with the loop button & loop brackets before realizing the cmd + l loop shortcut works wonders in the piano roll. just select the section you want to loop (check the teal bar above the note editor to confirm the selection), press cmd + l, & your midi clip is ready for you to extend as far as you please.

this is really helpful in making variations - you can loop out a smaller segment & use cmd + j on the resulting extended clip to make a larger loop. change a few things & you've got a more dynamic loop, wham!