Same thing for rearrangeable tabs with close buttons. I think Firefox introduced that concept (maybe it was Chrome), but it never got an official API, either on

Same thing for rearrangeable tabs with close buttons. I think Firefox introduced that concept (maybe it was Chrome), but it never got an official API, either on Mac OS X or Windows, which is strange because it's been such a fundamental part of certain programs for 15-20 years now. Lately macOS kind of has this, by allowing automatic grouping of documents for you, but it just feels hacked on compared to giving us this as an actual GUI component.