These are a few suggestions which I'd like:
First the most important: Perhaps I've just not figured out and there already is, but there should be a way to replay accords (all notes at a time), so you can hear your accord and not just the individual notes by clicking them. Perhaps clicking in the gray bar in the bottom on which a double-click triggers play could cause the notes at that time to be played.
Oh, and à propos double clicking there, double clicking again should stop the replay, if possible.
Placing notes should be more like in LMMS, that is: The current key/pitch on the left should be highlighted (the whole row in which the cursor currently is should be highlighted for better orientation), now only when the mouse is already clicked.
It should also not be necessary to explicitly change to another tool (move, stretch). Like in LMMS (e.g.) you could resize the note by grabbing it at its right or left end and move it by simply dragging it.
Clicking onto a note should play it. Currently, the note is only played when it is first selected. Any further click onto it has no effect.
Great program, by the way.