Quoting Ofnuts <ofn...@laposte.net>:
> Is there a practical, fast way, to detect this case, i.e, that the
> selection covers the whole layer (or image?) and that no pixels remain
> unselected? I assume that inverting the selection again and testing for
> empty would work, but that would be two selection inversions in the
> normal case and that may be a bit costly.
The following appears to be about twice as fast as the two inversion approach.
(let ((count (cdddr (gimp-histogram (car (gimp-image-get-selection image))
(= (car count) (cadr count)) ; compare num of all pixels to all white pixels
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