Add ability to unselect a radio button

[pingfan-hu]

Currently, if an mc or mc_buttons is clicked, it cannot be unchecked, unless the participant decides to select another radio button.

There are 2 possible solutions:

1. Add JS code to enable unchecking upon a 2nd click;
2. Add a button on the bottom right corner to clear selection.

I intend to try the 1st solution since it doesn't add anything in the UI. If it's not possible I'll move on to the 2nd solution.

[yihui]

The first solution is definitely possible and actually quite simple in JS: you just select the radio button (e.g., via const el = document.querySelector(...)) and then set the attribute el.checked = false. You may want to add a tooltip to the button group so users are aware of this possibility since this is not how radio buttons usually work. The second solution does clutter the UI a little bit (you can hide the clear button via CSS when :not(:hover)) but should be be more intuitive. I don't have a strong opinion on which solution is better. Note that for the second solution, you should be able to dynamically add the clear button via JS rather than pre-generating the button from R.

For the record: I came from yihui/yihui.org#1555 (comment) and yihui/yihui.org#1562.

[pingfan-hu]

Thanks @yihui for the advice! I'll try both and see if I can get a comfortable solution. Our goal of designing surveydown is to make it easy and intuitive for both survey designers and participants.

I might end up with both: a JS-generated clear button (and yes it's more intuitive) at the corner while also enabling un-toggling.