Interesting...
This project was for testing drum-related features in X2. I first recorded both test clips via MIDI, and then started experimenting with drum maps and the step sequencer. I guess I forgot to set the default (double-click) editor for the second clip back to piano roll.
I'm relieved that there is a workaround. However, it seems really strange that the choice of default editor has the odd
side effect of preventing edits in the other editors (at least the piano roll).
I think this is a bug rather than a desired feature, because there doesn't seem to be any advantage in preventing edits in editor A if editor B is the default editor. That would be like preventing edits in the Event List if the Piano Roll is the default editor, and Sonar has never worked that way.