I think you misunderstand what he’s asking. He doesn’t want right-to-left. He wants the export to render only V1 until the end of V1, then return to the beginning and render only V2, then return to the beginning and render only V3, etc., which basically means concatenate tracks instead of stack or layer tracks.
Basically, the playhead would be zig-zagging its way through the timeline, one track at a time, from bottom to top with his suggestion. A timeline screenshot would be difficult to interpret with this feature.
However, there is another way to satisfy his request without any code change at all. Instead of putting clips on V1 V2 V3, just throw them into the playlist then export from Playlist. It does the same thing he is requesting.
My understanding is that the OP wants a single export file (e.g. all_tracks.mp4) that consists of the clips on V1, followed by the clips on V2, followed by the clips on V3…
So far as I know, Shotcut cannot do this, or is that what the “export from playlist” will do?
Yes, I misunderstood that. The actual meaning sounds a lot simpler to me, I could have written a python script for that, but I don’t have enough time these days to work on making my program better compatible reading and editing the mlt of shotcut for doing this specific thing he wants. And transition, hmmm… A good idea to implement in my the to-do list for my mlt editor (just a basic python program to edit and send export the mlt through my personal export engine of shotcut). But I do already have the ability in it to export all tracks together as a column simultaneously, adding the ability of transitions between them can be easy (not for me, but for my hired and trusty dude).