Here’s a stack that works for me
Background video on primary story line
Fill video on a connected clip above the primary story line
Key source on a connected clip above the fill video. This clips blend mode is set to ‘stencil alpha’.
(The above is shown in the first screen grab. The next step is shown in the second screen grab which is before I made it a storyline.)
Make the fill and key clips a compound clip and then make the compound clip a secondary storyline to add a trans in/out. Does this help?


NT