The IBO FAQ page has two buttons:

<Reset previous Search> and <Submit Query Request>

Pressing <Reset previous Search> brings the last item
selected in the listbox to the focus.

I can find no way to clear it.

Selecting another topic in one of the other listboxes
and pressing <Submit Query Request> returns the
red-lettered statement below the first listbox:

Windows-standard multi-select is available in all lists.
Remember to clear selection each time and select at least one item.

I'm certain <g> that I must be the only one who doesn't understand
how to use this because it's been like this for a few years at least.

I am oblivious about how to 'clear' the selection.

The only way I find to manage this is to close the page, clear the
cache and open the page again so that I can search on a new topic.