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

[Mattias Engdegård]

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.)