Transitions on an overlay track keep the layers below

A transition between two clips on an overlay track blacked out everything underneath it for the length of the overlap. Put a crossfade between two logos on their own track and the video behind them cut to black for half a second, then came back.

Each side of a transition is wrapped in a small tractor over a flat floor, so a pillarboxed source’s transparent padding cannot band through the blend. That floor was opaque black on every track, which also made the transition’s own output opaque. On the base track that is free, because nothing shows through it anyway. On any track above it, it was the whole picture.

Only the bottom-most visual track flattens onto black now. Every track above it keeps its alpha and composites the way its clips already do outside a transition.

Nothing else about transitions changed: a base-track crossfade compiles to byte-identical output.

This one predated shape clips, and it turned up while testing them, because a bar or a badge crossfading on its own track is an ordinary thing to want.

The compose job reference covers how tracks composite, and the compose API page has the wider tour.