Tree: hierarchical focus and expansion

A project browser shows a Source folder, its App.ts and config.ts files, and a root README.md. Collapsing Source should hide its files and remove them from arrow navigation, while leaving the README reachable. This requires the visible hierarchy and the navigation metadata to agree. We will construct that agreement explicitly. Tree tracks expansion and keyboard activity here; opening or selecting a file remains an application decision. For navigable columns such as file size, continue from this guide to TreeGrid.

Represent the same hierarchy in metadata and DOM

The parent item’s hasChildren, the child’s parentId, and the group’s parentId must agree. level expresses the accessible depth. A group hides descendants when its parent is collapsed, while registration metadata lets keyboard traversal exclude those same descendants.

import { html, component } from "@typed/template";
import * as Tree from "@typed/ui/Tree";

export const ProjectFiles = component(function* () {
  const state = yield* Tree.makeState({ activeId: "files-source", loop: false });
  const collection = yield* Tree.makeCollection();

  return Tree.Root({ state, collection, label: "Project files", content: [
    Tree.Item({ state, collection, id: "files-source", level: 1, hasChildren: true,
      content: html`Source${Tree.Group({ state, parentId: "files-source", content: [
        Tree.Item({ state, collection, id: "files-app", parentId: "files-source", level: 2,
          content: "App.ts" }),
        Tree.Item({ state, collection, id: "files-config", parentId: "files-source", level: 2,
          content: "config.ts" }),
      ] })}` }),
    Tree.Item({ state, collection, id: "files-readme", level: 1, content: "README.md" }),
  ] });
});

This example is a hierarchy browser. Focusing App.ts does not open an editor or claim the file is selected. Add an explicit application action only after deciding whether activation means opening, selecting, or previewing. For a larger tree, keep the same ID graph in a domain model and derive both grouping and registration metadata from it; independently assembling them invites mismatches.

Traverse only the visible hierarchy

Root focus initializes the first item when there is no active ID and moves real focus to it. Up/Down and Home/End traverse visible registered items in DOM order. Right expands a collapsed parent; Right again on an expanded parent moves to its first child. Left collapses an expanded parent, otherwise moves to its parent. Enter/Space activates the active enabled item’s click behavior. The implementation does not add APG’s optional typeahead or expand-all-siblings command.

Expansion is stored by item ID. expand, collapse, and activate are public state transitions; calling them directly does not promise a DOM focus operation. The keyboard handler couples the necessary focus movement to those transitions. Ordinary item click updates active identity; it does not automatically toggle expansion. If your design wants pointer expansion, provide a deliberate control and reconcile its keyboard interaction with the tree instead of assuming click expands.

The APG tree view pattern distinguishes focus, selection, and expansion. Typed exposes aria-expanded for parents and aria-level for items but no built-in selection model. Adding aria-selected requires real application selection behavior; multiple selection needs more than an array of checked CSS classes.

Hidden descendants and removed descendants differ

Group uses the hidden attribute, so collapsed content remains mounted. Its registrations persist, but parent expansion metadata filters it out of visible keyboard traversal. This retains local state and avoids remounting; it also retains subscriptions. Unmount descendants separately if collapse should release their work.

Never use cyclic parent relationships. Visibility checks walk ancestors, so a parent graph must terminate. A missing parent or inconsistent hasChildren can make a visibly indented item behave like a root. On programmatic collapse, first move active identity/focus out of a descendant that will become hidden. Keyboard Left already acts from the currently focused node, but an unrelated collapse button can hide that node from elsewhere.

Nested content needs careful event ownership

Groups contain child treeitems, but arbitrary editors are not automatically integrated. Tree root keys can bubble from descendant controls. Do not embed a text editor and expect Left/Right to edit text while the tree independently handles the same event. The existing family has no edit-mode entry/exit protocol. Even nested click actions need testing because a child’s click bubbles through ancestor item elements; avoid attaching unconditional open-file actions to every ancestor.

Check expand, descend, return, and collapse with document.activeElement, expanded IDs, and hidden together. Also check programmatic collapse while a descendant is active: that path must repair focus. Public contracts: Tree API.