Super weird drag and drop/mouseMove/mouseRelease issue

[Geoff Canyon]

I'm working on extending drag and drop into/out of Navigator. To do that,
when a drag leaves Navigator I'm using a blendlevel'd stack to represent
the dragged objects as the pointer moves around the screen. This is in LC
8.1.8, on a Mac.

This works fine until the drag goes both to the right and below the list
field, and then the list field receives a mouseRelease message, and the
drag stops/fails.

Even weirder, it seems the stack representing the drag *starts* getting
mouseMove messages when the list field stops. I managed to tape together a
handoff routine that makes for a really odd workaround, but it's not a good
solution, obviously.

After some time whittling down the code to narrow down the issue, it ended
up being the field's horizontal scrollbar -- if it's set, the drag fails;
not set, everything's fine.

http://quality.livecode.com/show_bug.cgi?id=20957

(phew!)