When swapping out delegate of selectioncontainer, if the newly passed in delegate doesn't have any selectable content(which is usually the case), the selectioncontainerstate will notify all of the listeners. One of the listener would be SelectionRegistrant._updateSelectionRegistrarSubscription, and since it doesn't have content, it would remove itself from the listener which causes concurrent modification
* Can show context menus anywhere in the app, not just on text.
* Unifies all desktop/mobile context menus to go through one class (ContextMenuController).
* All context menus are now just plain widgets that can be fully customized.
* Existing default context menus can be customized and reused.
