leaky roof

I should mention this is a top floor apartment and the condo is responsible for fixing the roof.

if the leak is properly fixed, then nothing will happen to the value.

Are you talking about an apartment that's currently on the market? To me, it's a bit of a red flag to having something listed while the roof is leaking....

I generally agree with what The_President has said, but you may want to (1) determine if this has been a reoccurring problem, (2) get an independent assessment of the problem and the proposed solution, and (3) determine how quickly the condo has made repairs in the past as well as their timeline for fixing this problem.

I once lived in a place where a "minor" leak issue turned out to be the result of a serious design flaw, and the cost to remedy was about 10x the initial estimate and took three tries to get (mostly) right. Being paranoid about water infiltration every time it rains is a terrible way to live, so I'd be extra careful about roof leaks, however minor they may be.

The aprtment I am thinking of buying is not curently leaking. It had leaked about 3 years ago and was fixed.

if the roof hasn't leaked since it was fixed, and the cosmetic damage was fixed, it's not a problem. It could indicate a fairly "laidback" approach to maintenance (ie the board waits until something breaks before stepping in), so you may want to have a good inspector review the building.