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

[Drew Adams]

> For me mouse-3 is the preferred button to pop up a menu.  I never use
> mouse-3 to extend the selection because I'm not used to switch mouse
> buttons during one and the same action.

(Replying just about this: mouse-3.)

Vanilla Emacs hard-wires the behavior to kill or
delete the region (depending on the value of
`mouse-drag-copy-region').  That action can be
handy, but it's sometimes inappropriate (e.g., in
a read-only buffer).  In any case, it is only one
possible action; there are often other actions on
the selected text that you might want to take.

See `mouse3.el'.  You can use mouse-3 for both
a contextual pop-up menu and for extending the
selection.

https://www.emacswiki.org/emacs/Mouse3