bug#38013: [PATCH] Rectangular region selection with mouse

[Drew Adams]

> This is a proposal to add mouse-selection of rectangular regions. It
> turns out to be useful; Emacs should come with the facility built-in.
> 
> The main problem is what to bind it to. The common modifiers (shift,
> control, meta) are already taken. Platforms have different "standard"
> modifiers: Option on macOS, Alt on Windows, Control in Gnome (?).
> 
> Assuming that the secondary selection is somewhat of an anachronism
> which is likely to be less used today than the rectangular one would
> be, the patch uses Meta as default modifier. Both secondary and
> rectangular mouse selection have defcustoms permitting easy change.
> 
> The defaults could be different, for example if secondary mouse
> selection turns out to be very popular.
> There is no documentation yet.

FWIW -

1. I like the general idea of being able to use
the mouse to directly define a rectangular region.
_Good initiative_.

2. But I disagree completely that the secondary
selection is an anachronism.

IMO it is not used as much as it could (and should
and would) be used in Emacs only because it has no
_keyboard_ bindings, by default.

I use C-M-y to do all of this: 

* yank the secondary                 (no pref arg)
* select the secondary as the region (pref arg = 0)
* move the secondary to the region   (pref arg > 0)
* swap the secondary and the region  (pref arg < 0)

The 3rd and 4th of those set the secondary with just
the keyboard - from the region.  Another way to set
it using just the keyboard is to use C-x C-M-SPC to
start it and C-x C-M-<return> to end it.

(And I use C-M-y during Isearch to yank the secondary
to the end of the search string.)

The secondary selection is different from the region.
Its advantage, and its disadvantage, is that it is
liberated from point - it need not even be currently
visible in a window.  It doesn't change just because
you move the cursor around.

The region has a ring, `kill-ring', to let you get
previous selections.  I do the same thing for the
secondary selection, as well - give it a ring.  And
if the previous command yanked the secondary then
M-y yank-pops the secondary ring (not the kill-ring),
to replace that yank with the previous secondary on
the ring.

https://www.emacswiki.org/emacs/SecondarySelection#second-sel.el