[MlMt] Scrolling of message contents gets "stuck"

[Steve Burling]

MacOS 12.2.1 on a M1 Macbook Pro, MailMake release 5874 (and some 
earlier releases, too, so this isn't something new to 5874).

I've noticed in the last few (test) releases that sometimes I'm 
scrolling down through a message (using the trackpad on my MBPro, for 
whatever that's worth) and will come to a point in a message where I'm 
pretty sure that there's more message to be shown, but can't seem to 
scroll to it. Dragging up on the trackpad with two fingers just behaves 
as if it's at the end of the window. If I switch to a different message, 
and then come back to the one that previously got stuck at the end, I 
can usually continue to scroll past the point where I got stuck before. 
Odd.

This is in the "Thread Arcs" view.

-- Steve