Errant mouseRelease message?

[Geoff Canyon]

The drag and drop code in Navigator relies on mouseMove messages to do its
work, and since drags can exit the field where they started, mouseRelease
messages to stop.

When I first coded this, I sometimes got errant mouseRelease messages --
the mouse was still down, a check of "the mouse" in the mouseRelease would
report that the mouse was down, and yet the mouseRelease was still called,
and no more mouseMove messages were delivered.

I fixed that, and I don't know how.

Now the problem has cropped up again, and a quick scan through the diffs
(thanks, Git and GitKraken!) shows nothing that looks promising to have
caused this issue.

Any ideas? I'm on a mac, latest OS, LC 8.x