[grokys]

What does the pull request do?

Implements IExpandCollapseProvider on TreeViewItemAutomationPeer. A tree item can now report whether it's expanded, and can be expanded or collapsed through accessibility APIs.

What is the current behavior?

The peer implements only ISelectionItemProvider. A screen reader can't tell whether a node has hidden children. An automated test can only expand a node by clicking its toggle with synthesized input.

What is the updated/expected behavior with this PR?

• Expand() and Collapse() set IsExpanded.
• ExpandCollapseState is Expanded or Collapsed from IsExpanded, and LeafNode for an item with no children.
• Expand() and Collapse() throw InvalidOperationException on a leaf item, as WPF does. They throw ElementNotEnabledException when the item is disabled.
• A property-changed event is raised when the state changes. That covers changes to IsExpanded, and children being added or removed (e.g. LeafNode → Expanded).

How was the solution implemented (if it's not obvious)?

Expand() and Collapse() use SetCurrentValue rather than a plain setter. A common MVVM pattern binds IsExpanded from a style:

<Style Selector="TreeViewItem">
  <Setter Property="IsExpanded" Value="{Binding IsExpanded}" />
</Style>

A plain setter would set a local value. That would shadow the style binding for good, so the item would stop following its view model. TreeViewItem's own keyboard and pointer handling already uses SetCurrentValue for the same reason.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Fixed issues

Fixes #22460

🤖 Generated with Claude Code

https://claude.ai/code/session_01GK2AhwrPmuRp98cikjR2Sk

[Copilot]

🟡 Changes recommended

Reentrant child updates can produce incorrect state notifications, and Linux accessibility mappings expose invalid expansion actions for leaf items.

2 open issues

• Exclude leaf items from expandable AT-SPI states and actions · New
• Avoid duplicate expansion notifications during reentrant child population · New

What changed in this PR

Adds expand/collapse automation support to tree items, addressing #22460 so accessibility clients can query and change their expansion state.

Changes:

• Implements expansion, collapse, and leaf-state reporting with disabled-item checks.
• Preserves bindings through SetCurrentValue and emits state-change notifications.
• Adds unit tests for states, operations, exceptions, and notifications.

File	Description
tests/​Avalonia.Controls.UnitTests/​Automation/​TreeViewItemAutomationPeerTests.cs	Tests expand/collapse behavior and automation notifications.
src/​Avalonia.Controls/​Automation/​Peers/​TreeViewItemAutomationPeer.cs	Implements the expand/collapse provider and state-change reporting.

_{Review effort: Balanced}

[github-actions]

NuGet packages

The packages for this pull request have been published (workflow run):

• Version: 12.2.999-cibuild0100899-alpha
• Feed URL: https://nuget-feed-all.avaloniaui.net/v3/index.json