In case this bug reoccurs, a test case is as follows:
Run Tests/Intuition/ScreenTest TOP=200
Then move the workbench screen down a little, but keep it behind the ScreenTest screen.
Tests/Intuition/ScrollerWin
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Fixed by SVN r49128 (and also r49125 for VESA).
In case this bug reoccurs, a test case is as follows: