Unable to conditionally hide or show a CiviCRM Options field when using eventid query parameter

Hmm, normal webform conditionals for checkbox groups works on individual checkboxes, so eg if I have a checkbox with Color: Red, Orange, Green.

The conditional i need to write would be Hide if Any of

Red is checked.
Orange is checked.
Green is checked.

CiviCRM Options seems to be providing multiple types - select, multiselect, checkbox, radio & don't provide the separate logic for handling the conditionals.

One way to fix your usecase might be to re-type the element to use checkboxes (Can be done from Edit Element page).
Save
Then modify the conditional to check for each checkbox option as mentioned above.

Does that help?

It doesn't help at the moment as I'm using a select field, not checkboxes. Even when I switch to a standard select list I'm not sure it works. I don't know if it would work with the standard query parameter completion that is in webform.

What I've done with a different site is create a webform handler that checks the #default_value has been set and then hides the select field. I've found that if it is hidden by default, #default_value doesn't get set.