I added a Reframe video filter that you can only add to the Timeline > Output for the next version (not yet in daily builds).
Added a Reframe button in Export > Video to make it more obvious
(I also added a Resample button with a warning dialog to guard the resolution and frame rate fields that people misuse instead of Video Mode.)
Clicking Reframe adds the filter to Output or switches to it:
You can drag this rectangle control around and resize it to even dimensions only (for codec and pixel format compatibility), but you cannot push any part of the rectangle outside of the frame.
It supports keyframes and motion tracking. Here, I tracked the white horse in the first clip only and then re-centered it.
There are presets for 9:16 (defaults), 16:9, 4:3, and Square.
Export automatically converts the project to use Size, Position & Rotate to achieve the result without actually affecting your project–you can save the Reframe filter on your project and simply disable it when you want to export without it. You can have multiple Reframe filters and the first disabled one is used on export.
The black bars you see in this embedded player are not part of the video!
Here it is on YouTube Shorts