Layout sometimes misses mouseUp event

[blondejamtart]

When panning around the layout, sometimes the mouseUp event is missed and the layout gets stuck panning with the mouse. This is possibly due to mousing up outside the layout div.

[blondejamtart]

After further investigation, this seems to be linked to #408. Firstly, the defensive code from this bugfix was apparently lost between implementation and release but has now been re-implemented.
Additionally, this bug and the fix behave differently in firefox: dropping highlighted text anywhere on the page causes the browser to try and navigate to the text as a url. It is also possible, through some combination of panning the layout and/or attempting to pan again but clicking in or near the selection area of a link, to drag every link on the layout or the "selected" highlight of a link; dropping this causes the behaviour noted above (n.b. other things may also cause this behaviour)