Errors in CAPI display callbacks are problematic for the Debugger tool, because they can be invoked repeatedly. In order to handle this situation, the display of a CAPI pane where an error occurs in a display-callback (a "broken" pane) is normally disabled until the Debugger tool exits. Therefore while debugging such errors some panes will not be displayed correctly.
This issue can also occur with focus related callbacks, such as editing-callback in capi:text-input-pane.
If it is not easy to fix the problem, exiting the Debugger tool allows the error to happen again. To prevent this, in some cases there is a restart to disable the display of the broken pane permanently. Once this happens, the pane is not displayed correctly.
Once you fix the error, you can restore the display by capi:pane-restore-display, or by finding it in the Window Browser tool, and selecting the menu command
Works > Windows > Enable Display
.
LispWorks IDE User Guide (Windows version) - 13 Sep 2017