With the LiteHTML plugin loaded, vertically scrolling a message that has a horizontal scrollbar causes the vertical scrollbar to alternately disappear and reappear with each scroll of the mouse wheel. Messages without a horizontal scrollbar don't seem to be affected. You might need to enable loading remote content to force the horizontal scrollbar to appear.
Same root cause and fix as 4499 *** This bug has been marked as a duplicate of bug 4499 ***