When i first started charting, i used to run out of memory in reaper and i had no idea why. all i did was double click one track and every single track of a midi would open. then i realized, when i first loaded a midi into a project, EVERY track would be highlighted as selected. so if i clicked on one track, every track would open. i kept clicking on the next track, not realizing every track was selected still and then every track would open again, until it finally ran out of memory in reaper and crashed. in short, if you select more than one track, then look at ONLY one track, you will see those dark grey notes, as it’s loading every track you have selected but can only view one track at a time.