A tree node is a TreeViewItem: a header and items, patched by key as a
list's items are. The chosen node is chosen by its key, once the nodes are
there, and a click sends the key. SelectedItem is a direct property, which
a clear does not reset, so no choice is set as null.
The Readme now covers menus and keys, dialogs, styles and themes, and the
new controls. docs/styles.png is examples/styles.bjo.
🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)
Co-Authored-By: eca-agent <git@eca.dev>
