bug#38431: 27.0.50; mouse-drag-region-rectangle needs transient-mark-mode

[Mattias Engdegård]

Thank you, I've added a variant of your suggested change and pushed to master 
(9f2145f42d).

This does not really cure the special case you mention, but unless I'm 
mistaken, linear text selection with the mouse exhibits a similar (identical?) 
quirk. I'm not sure if it's a problem in practice since it requires a specific 
sequence of events.

Is the change good enough for everyday use? If not, please help elaborate, and 
if you think that linear text selection needs attention.