Other - Drag events and undo

Everything a drag goes through, as it happens, in the log below. The provider's OnDragStart and OnDragEnd run for every drag of its context, whichever chip it is and whichever input makes it - Native (the mouse, through the browser's drag and drop), Pointer (touch and pen) or Keyboard - and the end carries DidDrop and the result the tray's OnDrop published, which is all an undo history needs: Undo puts the last move back. Each tray reports OnEnter and OnLeave as a drag moves onto it and off it. A closed tray is Disabled: it takes nothing - no hover, no drop - and the keyboard skips it. The line at the top reads the drag state from outside the provider, through the manager the page hands it with Manager, whose StateChanged event tells the page when to look again.