[owenjonesuk]

Firefox implemented something a few years ago where buttons on pop-ups were greyed out for a few seconds, to avoid this. I believe it was a security concern that an online game could get you to click in a certain place at a certain time, and make a pop-up just before it asking for permission to do something, so you clicked Accept without meaning to.

[digi_owl]

They also have something like this on the download dialog, where the accept button stays gray unless the dialog has focus.