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

["Mattias Engdegård" <mattiase@acm.org>]

23 nov. 2019 kl. 12.57 skrev Eli Zaretskii <eliz@gnu.org>:

>> +@defvar fine-grained-mouse-movement
> 
> I think this variable's name should start with "mouse-".  Like
> mouse-fine-grained-movement or something.

True, but since the variable pertains to the 'mouse-movement' event in particular and not to moving the mouse about in general, it is also useful to have the name reflect that relation.
'mouse-movement-fine-grained' is possible but a bit awkward. What about 'mouse-fine-grained-tracking'?

>> +    (setq fine-grained-mouse-movement t)
> 
> What happens if this function signals an error? won't
> fine-grained-mouse-movement be left at its non-nil value?

Good question. Ideally, the variable (and 'track-mouse') would be dynamically bound during the drag. However, since the function, mouse-drag-region-rectangle, returns as the drag commences, this is not possible.

The code follows the pattern of other functions in the same file in that it trusts the exit function of the transient map to be executed eventually, and the first thing done in that exit function is to reset the variables.

All these functions have a small hole between setting the variables and the successful return of set-transient-map, during which an error will not result in the variables being reset. We could wrap the set-transient-map calls in condition-case everywhere, if we think that this would increase robustness. Basically,

(condition-case err
    (set-transient-map ...)
  (error (setq fine-grained-mouse-movement nil)
         (setq track-mouse nil)
         (signal (car err) (cdr err))))

An earlier version of the patch used the 'track-mouse' macro instead, which does not have this problem, but I changed after complaints that it does not permit independent use of the mouse click event. (For instance, if the selection uses M-down-mouse-1 and M-drag-mouse-1, M-mouse-1 could not be used for anything else.) However, I'm not convinced that this would be much of a limitation.

(Some code in mouse.el uses the track-mouse macro: mouse-drag-and-drop-region and mouse-drag-secondary.)