I split the edit window into two.
I dragged the second tab to another monitor.
The new window had a left-hand pane with several tool windows - I started clicking [X] because I just want the edit window.
When I tried closing "Symbols", the following SlickStack ocurred.
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Slick-C Error
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Invalid object handle
file=tbclass.ex offset=1106
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OK
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Invalid object handle tbclass.ex 1106 tbclass:_ClassFocusTimerCallback(221) p_window_id: 5 p_object: OI_FORM p_name: