Depends on D20383. Ref T13272. Fixes T12363. This gets the edit flows for tab panels functional again. They aren't //nice//, and a lot of the workflows are fairly janky: for example, most of them end up with you on the tab panel's page, which isn't useful if you started on a dashboard page.
However, these flows were extremely janky before anyway (see T12363) and I suspect this is a net improvement even though it's a bit of a mess. I anticipate cleaning this up bit-by-bit in future diffs.