Fixed size is usually not a good idea, why are you doing it?

Maybe activated makes more sense than clicked so it can be used with the keyboard too?

Why a readonly textedit instead of a label?

Same question as above

I'm not sure but i think it's a bit drasting closing this dialog when asking for "Signature Properties", what's the rationale behind it?

So how do you know 450x400 is going to fit with my big fonts and the text in Catalan instead of English?

It's interesting, if you look at Firefox, it uses a label in one place and what seems to be a textedit in another, meh.

Yeah.... let's not copy bad UI behaviour