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

[Stephen Berman]

After updating from master I wanted to try the new
mouse-drag-region-rectangle command but it didn't work with my init file,
and I found the reason was I have transient-mark-mode customized to
nil.  To reproduce:

0. emacs -Q (transient-mark-mode enabled)
1. Type `C-M-mouse-1', drag the mouse and a retangle gets selected.
2. M-x transient-mark-mode (now disabled)
3. Type `C-M-mouse-1', drag the mouse and no retangle gets selected.

I found no indication in NEWS, the Emacs manual or the doc string of
mouse-drag-region-rectangle that it is not supposed to work with
transient-mark-mode disabled, so I assume this is a bug.  This patch
makes it work with t-m-m disabled:

diff --git a/lisp/mouse.el b/lisp/mouse.el
index f076e90bd9..fd0ef2a360 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -1999,6 +1999,8 @@ mouse-drag-region-rectangle
         (unless dragged
           ;; This is actually a drag.
           (setq dragged t)
+         (unless transient-mark-mode
+           (setq transient-mark-mode 'lambda))
           (mouse-minibuffer-check start-event)
           (deactivate-mark)
           (posn-set-point start-pos)

However, this is not the right, or at least the complete, fix, because
it has some bad side-effects: after disabling t-m-m and then selecting a
rectangle with the mouse, now typing `M-x transient-mark-mode' still
says it is disabled; after typing it again, it is now enabled, but
typing C-SPC and then moving point does not show an active region; only
after typing C-SPC C-SPC and then anything (e.g. C-g) and then again
typing C-SPC and moving point is an active region shown.  So a better or
more complete fix is needed.

In GNU Emacs 27.0.50 (build 26, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo 
version 1.16.0)
 of 2019-11-29 built on strobe-lfs84
Repository revision: b2790db049da98b541d07bac21ca7d7c220d3be0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Linux From Scratch

Configured using:
 'configure --with-cairo 'CFLAGS=-Og -g3'
 PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS JSON PDUMPER LCMS2
GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix