Looking great so far! Non-rigorous testing looks good so far, we’ll see if I can find any edge cases.
I recently realized that “all day” calendar events can’t be rescheduled by drag/drop. Trying to put them in a different days’s “all day” area doesn’t stick, and dragging them into the hourly area of a different day leaves behind a big, day-long event block that hangs around until you horizontal-scroll the Pane or switch away/back. Calendar bugs are actually one of my more numerous categories, maybe that’d be a good area to get into next.
The one drag/drop behavior that most bothers me is auto-expanding on drop/paste as @LauraH just posted – my comments at that thread.