Instituted "cautious edge redraw".
We had the following problem, when double-clicking
on an edge segment:
On mouseup, the redraw command at the end of fuseBPs
was deleting the SVG object that was clicked on, so that
that object was gone before it ever received the click event.
Solution: redraw is now "cautious", in that it first checks
whether any of the edge's points have changed. If not, it
simply returns, without redrawing anything.
Now, when you double-click on an edge segment, that segment
can actually receive the double-click, and process it.