Disclose mental health issue during PhD during an interview? I am about to face an interview for a one year fellowship.

One of the criteria for the selection is length of time as a doctoral candidate.

While I am within my registration period, I'm allowed one additional year to submit my thesis and I'm about halfway through that very year.

Part of the reason for being "late" - i.e. taking the extra year to write which is in fact vary common despite being somewhat frowned upon by the programme I'm in - is that I had a long stretch of depression during my PhD which is better now.

Should I mention this during the interview?

Should I mention this only if they ask me about why I'm taking this time to finish?

Should I NOT mention this, but maybe hint at it in-between the lines?

Also, there's a non negligible chance the panel might be sympathetic toward the topic (i.e. I guess they might be people who I would deem more likely to take seriously the topic of mental health in academia, rather than having an opposing stance like "you should toughen up" or a dismissing one like "no big deal".)
I would suggest avoiding specifics unless pressed. "Personal reasons" should be enough for most. But the question may not even come up. There are enough reasons to miss "deadlines" in any sort of research program that most people won't give it much weight, unless very extended. Of course we like to finish things in a timely way, but the very process of research is inherently uncertain. 

You could even avoid the personal by citing "unforeseen obstacles" as your first response. It is fair to say "Personal" as the second version and "Health" as the third. Anyone pressing beyond that is being unfair as long as you assure them that the situation is in the past and not recurring.