bug#51557: 27.2; `mouse-1' with `cua-rectangle-mark-mode'

[Drew Adams <drew.adams@oracle.com>]

I don't use `cua-rectangle-mark-mode'.  I just happened upon this
behavior, and found it odd (bugged?).  Feel free to close if you
think this isn't problematic.

emacs -Q

M-x cua-rectangle-mark-mode ; (after selecting a region or not)

Click `mouse-1' somewhere.  No effect, it seems.  The mode remains in
effect (same key help is displayed).

Click `mouse-1' again (anywhere).  It seems that that exits
`cua-rectangle-mode'.

Should `mouse-1' do something for this mode?  (`mouse-3' apparently
extends the rectangle.)  Should it exit on first click?  Should you get
some feedback of whatever it does?

`rectangle-mark-mode' also lets `mouse-3' extend the rectangle.  But for
it, `mouse-1' apparently exits the mode.

That seems more reasonable.  But perhaps the mode should be continued
and `mouse-1' should just reinitialize it (set its starting point).
(The normal behavior of `mouse-1' is, after all, to set point.)

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19042
System Description: Microsoft Windows 10 Pro (v10.0.2009.19042.1288)