bug#16403: 24.3.50; Rectangular selection visually disturbing

[Bastien]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>>> I'll will wait for other maintainers to give feedback and apply
>>>> this on monday if it's good.
>>> It's probably fine for ttys, indeed, but it doesn't solve the problem
>>> for GUI frames.
>> I thought the behavior in GUI frames was acceptable,
>
> This OP's use-case is a GUI frame.  Daniel pointed out that it's even
> worse under ttys.

Okay.

>> Maybe in GUI the visual hint could be a change in the cursor
>> shape?  E.g. from box to vbar.
>
> The intention is not just to warn the user that we're in rectangle mode,
> but to actually show the boundaries of the 0-width rectangle.

I will commit the change about the warning independently of the way
rectangle boundaries are shown.

Two ideas:

1) using a vbar cursor, which not only warn the user, but also
   visually represent an empty rectangle.

2) reduce the width of the line inserted to reduce the glitch.

(Both are compatible, of course.)

Note that vim does not allow empty rectangle selection:
C-v y p will copy and paste the character at point.

I cannot think of situations where the user would want an empty
rectangle, but at the same time not allowing them would not be
consistent with Emacs behavior.

-- 
 Bastien