This is probably an Opera bug, but it would be nice to have a Bugzilla workaround. Using Opera 5.11 under WinNT4SP6a. On the Bugzilla query page, I can't deselect all items in a listbox, so I can't create queries that ignore some fields. Opera opens the page with the first item in all listboxes selected.
Problem continues with today's Opera 5.12. I researched this a bit at W3C and see in HTML v4 that it's up to the browser vendor what to do when encountering a <SELECT> with no default selection. The browser may elect to select the first <OPTION> or none. So I think Bugzilla should provide an "any" or "none" option in all menus where appropriate. Reference: http://www.w3.org/TR/1999/REC-html401-19991224/interact/forms.html#h-17.6 Additionally, the pages lack DTD tags at the top, so I have to guess what DTD to validate against using the W3C validator (http://validator.w3.org/). I recommend adding a validator button to the bottom of all bugzilla pages so you can quickly check whether you have valid HTML before deployment. An example of this button is present at the bottom of the W3C validator page.
What if you use the control-key when selecting a item in the listbox? It should de-select the item. I tried this with Opera 5 for Linux and Mozilla with similar results.
Red Hat's current Bugzilla version is 2.18. I am moving all older open bugs to this version. Any bugs against the older versions will need to be verified that they are still bugs. This will help me also to sort them better.