tree: emit TreeEvent on expand/collapse

[jayson-saavylab]

(No associated issue - small additive enhancement)

Description

Add explicit TreeEvent to TreeState so consumers can react to expand/collapse changes without maintaining a shadow copy of expansion state.

For lazy-loading trees (file browsers, database explorers, remote object browsers), consumers currently need to cx.observe(&tree_state, ...) and diff the entire expansion set after every notify.

With this change, consumers can subscribe directly:

cx.subscribe(&tree_state, |this, _, event: &TreeEvent, cx| {
    if let TreeEvent::Expanded(id) = event {
        this.load_children_for(id.as_ref(), cx);
    }
});

Break Changes

Fully additive. Existing cx.observe(&tree_state, ...) consumers continue to work unchanged.

How to Test

cargo test -p gpui-component tree

Four new tests were added:

• test_emits_expanded_event - toggling a collapsed folder emits TreeEvent::Expanded(id)
• test_emits_collapsed_event - toggling an expanded folder emits TreeEvent::Collapsed(id)
• test_set_items_does_not_emit_expansion_events - rebuilding the tree via set_items() is silent
• test_event_carries_item_id - event payload is TreeItem.id, not row index

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• [N/A] Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) (not platform specific)

AI Assistance Disclosure

🤖 Parts of this PR were generated with AI assistance and subsequently reviewed, tested, and refactored by a human to match the project's existing code style and patterns. Specifically:

• The TreeEvent enum and EventEmitter implementation were drafted with AI, then aligned to match the existing TableEvent / SliderEvent patterns in the crate.
• The test cases were AI-generated and then verified by running the full gpui-component test suite (219 tests pass, including the 4 new ones).

[madcodelife]

Thanks for the PR!

One small detail though: If you use set_selected_item to select a node hidden under collapsed folders, it bypasses toggle_expand so no TreeEvent::Expanded is emitted — a lazy-loading subscriber would never load children for folders expanded this way.

[jayson-saavylab]

Thanks for the PR!

One small detail though: If you use set_selected_item to select a node hidden under collapsed folders, it bypasses toggle_expand so no TreeEvent::Expanded is emitted — a lazy-loading subscriber would never load children for folders expanded this way.

Hey thanks for the feedback, this should be resolved now! set_selected_item now routes ancestor expansion through the event-emitting path, so hidden folders fire TreeEvent::Expanded when revealed. I also reversed the ancestor iteration so events fire root -> leaf, which is the order a lazy-loading subscriber would like