Kate should have the same tearable tab ability as browsers, drag and dropping a tab from Kate into the space outside of Kate should open up the tab in a new instance of Kate. Currently Kate seems to want to copy the file instead which is unintuitive. Dragging and dropping a tab outside of Kate should default to 'detach document'. Also the reverse will add the tab to Kate, but the existing instance of Kate with that tab will stay open, it should close automatically.

This previous bug report talked about the added function https://bugs.kde.org/show_bug.cgi?id=460923 but it now needs to be added to the drag and drop behaviour (for both tear and attach)