Support radio fields without defaults

[fkohrt]

The documentation on radio fields says

Also the first command, which initializes the field, has to set the default value of the group.

[...]

If no default is set this is the first button.

Is the existence of a default a necessity from a PDF standard or a design decision for l3pdffield? If it is the latter, I would find it very helpful to also have the option of not having any radio in a group pre-selected, just like in HTML forms. If I receive a filled form, it is helpful to know whether users missed a particular question or found no option to be fitting, which is hard to tell if the first option is always pre-selected.

[u-fischer]

As far as I remember, it is a design decision. At least the docu says

It is theoretically possible to control the start appearance state for every button so
e.g. all buttons could have the “selected” state when the PDF is opened. ...

The problem is that if your undecided user clicks by accident on such a field they will not be able to revert to the "all unchecked" state. If you want to support such a case I think it will be much cleaner if you add an explicit button "no answer" and make that the default.

[fkohrt]

they will not be able to revert to the "all unchecked" state

(Though it appears that's a bug with the PDF viewers, when NoToggleToOff is ignored.)

I'd say if reverting the click is important, a reset button can be provided, like here.

I think it will be much cleaner if you add an explicit button "no answer" and make that the default.

That certainly works, though in a sense I don't find it cleaner to add one more radio to every group.

Maybe having an explicit nodefault key would work if you think that form creators should not be encouraged to do this, without making it completely impossible?