Finally figured it out.  I had overridden #handlesMouseMove: to answer
true and, for some reason, Morphic calls handleMouseStillDown: from
handleMouseMove: even when no button is down.  Strange!

On Fri, Apr 18, 2014 at 10:14 AM, Chris Muller <ma.chris.m at gmail.com> wrote:
> I have a custom morph that overrides handlesMouseStillDown: to return true.
>
> Simply by doing only that, it causes
> WorldState>>#drawWorld:submorphs:invalidAreasOn: to send
> #fullDrawMorph: with my Morph as the argument on every MouseMove event
> over my morph, regardless whether any button is down or not.
>
> I only noticed it because the rapid redrawing causes a slight
> flickering in my Morph.
>
> Setting handlesMouseStillDown: back to false, my morph's redrawing
> code is not executed at all when I move through it.
>
> I have no idea why simply handling this event causes the rapid redrawing.
>
> Thanks for any suggestions..
>