bug#31240: 26.1; mouse-save-then-kill does not kill rectangles

[Federico Tedin <federicotedin@gmail.com>]

> If I now drag it up by one line and four columns to the right (so that
> it just does not intersect with itself) I get
>
> ;;;;;;;;;;RECT;;;;;;;;;
> ;;;;;;RECT;;;;;;;;;
> ;;;;;;RECT;;;;;;;;;
> ;;;;;;RECT;;;;;;;;;
> ;;;;;;RECT;;;;;;;;;
> ;;;;;;;;;;;;;;;
> ;;;;;;;;;;;;;;;;;;;
>
> which is somehow counterintuitive.  Any ideas?

This is because after inserting the rectangle, the original rectangle
(which is to the left of the inserted one) is deleted. When it is
deleted, then the last 4 lines of the inserted rectangle are shifted
to the left:

1) Inserted new rectangle:

;;;;;;;;;;RECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECT;;;;;;;;;
;;;;;;;;;;;;;;;;;;;

2) Deleted the original one:

;;;;;;;;;;RECT;;;;;;;;;  <--- this line isn't shifted
;;;;;;RECT;;;;;;;;;
;;;;;;RECT;;;;;;;;;
;;;;;;RECT;;;;;;;;;
;;;;;;RECT;;;;;;;;;
;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;

I've thought up two ways of dealing with this problem:

A) Forbid the user from dragging the rectangle to the right when the
new rectangle is *not* completly above or completly below the original
one. As an exception, allow the user to drag the rectangle purely to
the right (same line). In this option, the drag operation in your
example would not be permitted.  I think this option would be a bit
confusing for users (and too restrictive).

B) Add a variable mouse-drag-and-drop-rectangle-fill, initially set to
nil. When the variable's value is nil, the behaviour of dragging and
dropping rectangles is exactly the same as it is now. When it is set
to a non-nil value (for example, " "), then replace all characters of
the original rectangle with the variable's value. In this case, your
example would look like this:

1) M-: (setq mouse-drag-and-drop-rectangle-fill " ")

2) Inserted new rectangle:

;;;;;;;;;;RECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECTRECT;;;;;;;;;
;;;;;;RECT;;;;;;;;;
;;;;;;;;;;;;;;;;;;;

3) Deleted the original one:

;;;;;;;;;;RECT;;;;;;;;;
;;;;;;    RECT;;;;;;;;;
;;;;;;    RECT;;;;;;;;;
;;;;;;    RECT;;;;;;;;;
;;;;;;    RECT;;;;;;;;;
;;;;;;    ;;;;;;;;;
;;;;;;;;;;;;;;;;;;;

I can't think of any other way of ensuring the inserted rectangle
retains its shape after being inserted, if there are pending delete
operations to its left.