After hiding operator area by moving border edge in tool shelf, small 'Plus' icon appears to allow restoring this area again.
After moving window border edge, operator area disappears but no 'Plus' icon visible.
Attached images and .blend file to test.
Ubuntu 10.04; Blender r35956.
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@ ronan ducluzeau - what do you mean always present? Is showing up in lower 3d view or always present in upper 3d view.
Previous .blend file has wrong setup (own mistake while saving it) so it may be confusing a bit.
Uploaded new .blend with proper setup so test it with op_area_bug_2.blend file.
Just tested official RC1 on WinXP-PRO-32 and like I said before, no 'Plus' icon after moving window border.
Issue confirmed & known. The code for these (+) icons has been badly implemented, in some cases they don't show, other cases they show in wrong corners.
The visualization is under review as well. I personally think it's not a good way to indicate window-manager features. These icons are used for UI widget visualization now too.
It is an old issue on the wiki todo as well, and high on the list to solve. Bigger bugs in blender still have priority though.