Mouse rectangular region selection (bug#38013)

Make it possible to select a rectangular region using the mouse.
The standard binding is C-M-mouse-1.

* lisp/mouse.el (mouse-scroll-subr): Add ADJUST argument.
(mouse-drag-region-rectangle): New.
* lisp/rect.el (rectangle--reset-point-crutches): New.
(rectangle--reset-crutches): Use 'rectangle--reset-point-crutches'.
* src/xdisp.c (remember_mouse_glyph, syms_of_xdisp):
Add 'mouse-fine-grained-tracking'.
* doc/lispref/commands.texi (Motion Events):
Document 'mouse-fine-grained-tracking'.
* doc/emacs/frames.texi (Mouse Commands):
* doc/emacs/killing.texi (Rectangles):
* etc/NEWS: Document rectangular selection with the mouse.

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/lispref/commands.texi b/doc/lispref/commands.texi
index 0c848a80257..032f005e9c4 100644
--- a/doc/lispref/commands.texi
+++ b/doc/lispref/commands.texi
@@ -1661,6 +1661,12 @@ events within its body.  Outside of @code{track-mouse} forms, Emacs
 does not generate events for mere motion of the mouse, and these
 events do not appear.  @xref{Mouse Tracking}.
 
+@defvar mouse-fine-grained-tracking
+When non-@code{nil}, mouse motion events are generated even for very
+small movements.  Otherwise, motion events are not generated as long
+as the mouse cursor remains pointing to the same glyph in the text.
+@end defvar
+
 @node Focus Events
 @subsection Focus Events
 @cindex focus event