Please contact your provider.
I did.
Are developers reading this forum? While the odd behavior was reproducible, the following JS code was executed (see my issue on github):
if (this.env.action == 'compose') { if (opener_rc && opener_rc.env.action == 'compose') { setTimeout(function(){ opener.history.back(); }, 100); } } // stripped some code including the call to init_messageform()
It looks like the "compose in new window" setting wass partly enabled. It does not open a new window, but does the cleanup in the original window. If this is a rare and volatile bug, then someone might be interested.