bug#24755: 25.1; Selection continues with no mouse button pressed

[Eli Zaretskii]

> From: Mats Palmgren <matspal@gmail.com>
> Date: Fri, 21 Oct 2016 16:56:00 +0200
> 
> 1. Visit a buffer with some text
> 2. Press the left mouse button
> 3. Drag the mouse to select some text
> 4. Press the Control key
> 5. Release the mouse button
> 6. Continue to move the mouse
> 
> Actual Result:
> Moving the mouse continues to select text, even though no mouse button
> or keys are pressed.
> 
> Expected Result:
> Selection should stop at 5.

Stefan, could you please fix this?  It seems to be due to changes in
how mouse-drag-track handles mouse events: in Emacs 24.5, the code
handled mouse clicks as well, whereas now it doesn't seem to.  So
releasing the mouse button while Ctrl or Shift is pressed signals an
error now, instead of finishing the drag event.

The fix should go to the emacs-25 branch.

Thanks.