I thought I always saw scrolling mentioned.
The test cases I made for Hoch had nothing but text rendering.
That's why I consider one of solutions is to speed up the text output... besides Carbon it using the deprecated ATSU API anyway.
So updating to conditional usage of CoreText might allow Carbon widgetset to survive another OSX release
Why conditional? CoreText is not available in OSX 10.4
This knowledge my be beneficial to Cocoa (if Cocoa suffers from the same issue).
IIRC, Cocoa is good enough to compile an application with SynEdit.
For scrolling issue - yes, please review ScrollWindowEx test case. The morel lightweight example is, the easier it would be to debug and fix.
Trying to reproduce an issue with SynEdit (with lot of it's settings and compiler conditions) doesn't seem easy enough to spend time on it.