... Do for release as was already done for click and drag: hold a shared
pointer.
This will protect against reentrancy surprises as in this bug, where a
reference to the handle may be removed during one of the handle's own
methods.
... while the project makes note of which window has temporarily given up the
focus; and TrackPanel can detect that, so that the yellow border of the focused
track is still drawn while the panel "lends" the focus.
Why do all this? So that, when the ruler is another CellularPanel, ESC key
will work to abort drags in the ruler, but TrackPanel appearance won't change
during the drags.
