The functions
capi:prompt-with-list-non-focus
and dispalys a list in a non-focus window, which does not take the input focus and hence does not see keyboard input in general. It responds to mouse gestures. Other APIs allow you to specify particular keyboard gestures that are seen, control a filter on the list, close the non-focus window and so on.
This mechanism is used in the LispWorks IDE in various commands which offer completion of input.
capi:text-input-pane-in-place-complete
uses this mechanism to provide in-place completion in a text input pane.