Add an anchor system to SuperEditor widget

[matthew-carroll]

It should be easier than it is to display popover toolbars and other overlay anchored items.

Currently, developers need to provide a ScrollController to SuperEditor so that the developer can update the toolbar's location on every frame. This is very tedious and it's likely to fail when there's more separation between SuperEditor and the rest of the app behavior.

One idea that I have to deal with this is to pass a map of DocumentSelection -> LayerLink. The SuperEditor widget can maintain a set of invisible SizedBox widgets at the locations and sizes represented by the various DocumentSelections and those boxes can be wrapped in a LinkTarget.

Then, builder widgets could be used to easily anchor other widgets in the overlay.

SuperEditorAnchoredBuilder(
  link: _selectionLink,
  overlayBuilder: (context) {
    return EditorToolbar(
      editContext: editContext,
    );
  },
  child: SuperEditor(
     //...
     anchors: {
       // Place an invisible anchor around the user's selection
       _selectionLink: composer.selectionNotifier,
     },
  ),
);

[jonaird]

Another issue I'm having that would need to be addressed by this system is that my toolbar is a bit large and it prevents the user from being able to scroll the document when the mouse is on top of the toolbar. This is because the toolbar isn't actually in the scrollable widget. afaik I don't think it's possible to collect raw scroll information?

Another use case is slash commands. For example in Notion, if you type "/h1" a filtered list of blocks shows up and hitting enter will create a new block of that type.