fix(treeview): typed data-driven TreeView<T> with per-container hosting (#447)

[codemonkeychris]

Summary

Closes #447 — a TreeView whose TreeViewNodeData nodes carry a ContentElement rendered blank rows. A node-mode WinUI TreeView stringifies TreeViewNode.Content and cannot host a pre-built UIElement; rich per-node visuals must come from a template (a data → Element function), never an element instance.

This adds a typed, data-driven TreeView<T> — the hierarchical peer of ListView<T>/GridView<T>/FlipView<T>:

TreeView(
    items,
    keySelector:      n => n.Id,
    childrenSelector: n => n.Children,        // the hierarchy
    viewBuilder:      n => /* data → Element */)   // the per-node "template"
// + IReactorKeyed overload that drops keySelector

Heterogeneous nodes with per-shape visuals are a switch inside the viewBuilder (the C# equivalent of WinUI's ItemTemplateSelector). OnItemInvoked / OnExpanding are Action<T> and hand back the developer's own T.

TreeViewNodeData.ContentElement is marked [Obsolete] pointing at TreeView<T>; the legacy path stays functional (CS0618 suppressed at the internal use sites).

Hosting — aligned with WinUI's model

Hosting mirrors the typed ListView<T>, which is how WinUI itself drives rich item content:

• The ItemTemplate is an empty ContentControl shell.
• Each node's view is mounted imperatively into the realized container via the internal TreeViewList's ContainerContentChanging — a fresh view on realize, unmounted on recycle. (We don't set args.Handled, so the TreeViewList's own handler keeps doing its indentation/selection work.)
• node.Content holds the developer's data item; the ItemInvoked / Expanding trampolines read T back from it.

This keeps expand/collapse robust under container recycling, fixing the "every other expand/collapse blanks the first child row(s)" regression that an earlier declarative {Binding Content} approach suffered (a recycled/collapsed container retained the element's visual parent, so a different pooled container couldn't re-host it).

Update reconciles only the realized containers' views in place, with a minimal-mutation node reorder so unchanged-order updates touch the live node collection not at all; unrealized nodes rebuild from node.Content on next realization. Full-tree unmount walks realized containers so each node-view Component's cleanups run.

Files

• src/Reactor/Core/Element.cs — TemplatedTreeViewElementBase (object-erased base so the reconciler switch matches one type) + TemplatedTreeViewElement<T>; OwnPropsEqual arm. Value-type T is boxed once; reference-type T flows through the covariant IReadOnlyList<object> conversion.
• src/Reactor/Elements/Dsl.cs — the two TreeView<T> factory overloads.
• src/Reactor/Core/Reconciler*.cs — empty-shell ItemTemplate, ContainerContentChanging hosting on the internal TreeViewList (found + cached per TreeView), keyed in-place node diff, full-tree unmount walk.
• src/Reactor/Core/V1Protocol/ChildrenStrategy.cs — CS0618 suppression around the legacy ContentElement reads.
• samples/Reactor.TestApp/Demos/DataTemplateDemo.cs — section 4 migrated from ContentElement to TreeView<T> with a discriminated PetNode model (distinct templates via switch).

Tests

tests/Reactor.AppTests.Host/SelfTest/Fixtures/TemplatedTreeViewFixtures.cs (TTV_): rich-content render, heterogeneous per-shape templates, in-place keyed reconcile, structural add, expand/collapse cycles stay rendered, event-erasure T resolution, value-type T, IsExpanded selector, unmount teardown.

• Builds clean on ARM64 (src/Reactor, TestApp, self-test host) — 0 errors, no CS0618 leakage.
• Full self-test suite green (4454 ok, 0 failures), including a fixture that drives several collapse→expand cycles and asserts every child row stays rendered.
• The headless harness renders at full window size and can't fully reproduce virtualization-recycle behavior, so the expand/collapse fix should also be eyeballed in the TestApp (DataTemplate Demo → section 4).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a typed, data-driven TreeView<T> that renders rich per-node visuals via a viewBuilder, aligning TreeView with existing typed collection controls and avoiding legacy TreeViewNodeData.ContentElement hosting issues.

Changes:

• Introduces TemplatedTreeViewElement<T> and DSL factory overloads.
• Adds reconciler mount/update/unmount support for per-container TreeView node hosting.
• Migrates sample/demo usage and adds self-test fixtures for rendering, updates, events, value types, expansion, and unmount.

Show a summary per file

File	Description
src/Reactor/Core/Element.cs	Adds typed TreeView element model and obsoletes legacy ContentElement.
src/Reactor/Elements/Dsl.cs	Adds TreeView<T> factory overloads.
src/Reactor/Core/Reconciler.Mount.cs	Mounts typed TreeView nodes and hosts realized container content.
src/Reactor/Core/Reconciler.Update.cs	Adds keyed hierarchical update/reconcile for typed TreeView.
src/Reactor/Core/Reconciler.cs	Adds typed TreeView container cache and unmount traversal.
src/Reactor/Core/V1Protocol/ChildrenStrategy.cs	Suppresses obsolete warnings for legacy TreeView content reads.
samples/Reactor.TestApp/Demos/DataTemplateDemo.cs	Updates demo to use typed TreeView<T>.
tests/Reactor.AppTests.Host/SelfTest/*	Registers and adds typed TreeView self-test fixtures.

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• Files reviewed: 9/9 changed files
• Comments generated: 2