13.5 How to draw to an on-screen port

Drawing on an output-pane should be done only during the dynamic extent of the call to the display-callback that LispWorks makes whenever part of the pane needs redrawing, See output-pane for more information about this initarg.

For other situations where you want to redraw the pane, you should call invalidate-rectangle or redisplay-element, which will cause the display-callback to be called.