RadioButton allows deselecting

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Radio button allows deselecting (when in a group).

This is different from native radio buttons, which do not allow deselecting a button by clicking it again.

You can do the comparision in the test_Checkbox.html (6838rev)

There should be at least an option to prevent this behavior.

[bill]

Good point, I will fix that.

[bill]

(In [6911]) Fixes #2174 (clicking a radio button that is "checked" shouldn't "uncheck" the radio button)

I removed the preventDefault() call for non-a11y checkbox onClick(); images don't have a default handler for clicks anyway. (They aren't like <a>) I don't see a reason for the stopPropogation() call either but I left it in for now.