Ijust went thru figuring this out too. I didn’t read the definition of “handles” closely enough, and because of past experience with other NLEs, I misunderstood this. I thought that just having plenty of frames to work with on either side of a cut point was enough, that the notion of ‘handles’ was simply having lots of frames. In the past, in other programs, I could just place a transition at the point between two clips and it would use frames on either side.
But with FCP, for a centered dissolve, you actually have to set an end point (at least) 15 frames from the end of the first clip, i.e. there are 15 frames that are ‘invisible’, that are not seen unless you add a transition. And, the second clip has to have a start point set 15 frames (or more) into the clip.
You have to edit each clip, setting end points and start points that, in a sense, clip off some frames, and those frames are then used for transitions.
This is something that is obvious to an experienced FCP person, but absolutely drove me bonkers for days, mainly because I had always just placed transitions in the past without having to do anything to clips that were adjacent to each other.