Hard to believe this isn't a dupe but searching with "pointer" or "compose"
didn't reveal anything. Anyway, to reproduce you have to:
1. Open a message compose window
2. Resize the window for the horizontal scrollbar to appear.
3. Put the mouse pointer on the scrollbar
Mouse pointer becomes what system specifies (in Win32, it is a white arrow)
Mouse pointer is the same as inside the content area (i.e. a vertical line).
i wonder if there's an older one too, but it's been reported.
*** This bug has been marked as a duplicate of 132851 ***
*** Bug 135122 has been marked as a duplicate of this bug. ***