After adding several Pip tracks i continued editing the main video track by adding transitions between clips... Then suddenly the drag/drop of transitions stopped working (a forbidden signal appears).
I discovered that this issue appears because the time track view was too height then i scrolled it down to keep music tracks visible. When you drop the transition to the main video track (still visible) PD badly check the drop target on the Pip track that is scrolled "under" the main video track.
I suppose this is a bug?
Umberto