[MlMt] hang on scroll w/spacebar

[Shoshanna Green]

As of today, I can reliably trigger a spinning-pizza hang by having a 
message open in its own window for viewing, and trying to scroll it with 
the space bar. Hit spacebar, MailMate freezes for a few seconds (mouse 
cursor doesn't respond to mouse movement), and then I get the spinning 
pizza, though that does move with the mouse so I can switch to another 
app, or go down to the Dock and control-click the MM icon ("Application 
Not Responding") to force-quit it.

Scrolling with the mouse wheel works fine, so I can still read long 
messages :)

I'm running version 5712 under High Sierra.

Shoshanna Green
shoshannag at gmail.com