This change collapses the selection at the clicked/tapped location on single click down for desktop platforms, and on single click/tap up for mobile platforms to match native.
This is a change from how `SelectionArea` previously worked. Before this change a single click down would clear the selection. From observing a native browser it looks like when tapping on static text the selection is not cleared but collapsed. A user can still attain the selection from static text using the `window.getSelection` API.
https://jsfiddle.net/juepasn3/11/ You can try this demo out here to observe this behavior yourself. When clicking on static text the selection will change.
This change also allows `Paragraph.selections` to return selections that are collapsed. This for testing purposes to confirm where the selection has been collapsed.
Partially fixes: #129583
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.
