bug#15960: 24.3.50; Provide a good way to tell when in `mouse-drag-track' (regression)

[Drew Adams <drew.adams@oracle.com>]

> > Emacs 24 changed things, so that this mode-line highlighting now
> > appeared immediately when you press `mouse-1', and did not wait
> > until you actually drag the mouse or release `mouse-1'.
> 
> Might simply be a bug.  I suggest you focus on tracking why this is.

It might.  I have no objection to it being seen as a bug, and fixed.

But the behavior of the highlighting appearing now before you release
mouse-1 is a definite improvement.  I would not wish to see that lost
because of the "fix".  This just should not happen unless at least one
char is selected.

In any case, Chong Yidong (I think) changed the code considerably for
Emacs 24.  It will take someone more knowledgable than I to figure it
out, let alone figure out just what was intended vs what might have
been accidental.

Providing a good, clean way to tell whether `mouse-drag-track' is in
progress (e.g. `mouse-tracking-p') would solve the problem I am
reporting.  I would guess that it might also be useful more generally.

But if you don't see it that way and don't want to fix this, I'll
continue to rely on (eq 0 echo-keystrokes) as the test for this.