mu4e~view-quit-buffer didn't set mu4e~headers-view-win to nil. Hence, mu4e~headers-move thought that the message view had to be updated even when the message view was actually closed. I tested with all three possible settings of mu4e-split-view.
Set mu4e-headers-view-win to nil when closing message view.
Merge remote-tracking branch 'upstream/master'
Also fixes #165. See comment there.