Rebuild the assembly-tree namespaces eagerly, as the WPF host did

[siegfriedpammer]

Problem

With Use nested namespace structure enabled, most namespaces are missing from the assembly tree, and the first expand of a large assembly lags. This is the same bug #3879 addresses.

Both symptoms share one root cause. The Avalonia assembly-tree nodes were written from scratch as lazy scaffolding rather than ported from the WPF host, and lost the single eager namespace build the WPF design (release/10.1) used:

• A NamespaceTreeNode only ever filters as Recurse/MatchAndRecurse, so the filter cascade computes its IsHidden as "all children hidden" — vacuously true for an empty child collection. The lazy build attached each namespace node while it was still empty, so intermediate namespaces that hold only sub-namespaces (e.g. System.Collections in an assembly that only ships System.Collections.Generic) latched hidden and stranded everything beneath them.
• The cascade force-loads every namespace node's children regardless, so the per-node laziness avoided no work — it rescanned the whole TypeDefinitions table once per namespace node (the lag, at ~17k types).

Solution

Restore release/10.1's structure:

• AssemblyTreeNode builds the entire namespace band in one pass over the module's top-level types, and populates each node before attaching it, so the empty-children IsHidden latch never fires.
• The band is built from the module's type system, like 10.1 — each TypeTreeNode holds the resolved ITypeDefinition it renders from, so painting a cell never re-enters the settings-keyed type-system cache. Master's lazy node did, on every Text / Icon / Filter read, rebuilding an effective-settings object and taking its lock each time.
• Ordering the pass by full ReflectionName is what interleaves each namespace's types and its sub-namespaces into one alphabetical run — a sub-namespace attaches when its first descendant type is reached, landing at its own alphabetical slot among the sibling types. That is the WPF order; grouping all types ahead of all namespaces was a visible departure.
• Two indexes — full namespace name → node, and type handle → node — keep FindNamespaceNode / FindTypeNode O(1) and correct at any nesting depth. TreeNodeLocator.FindTypeNode (hyperlink clicks, search-result activation, JumpToType) delegates to that index instead of walking children by display name, which never matched in nested mode.
• NamespaceTreeNode goes back to a dumb label holder and re-escapes its display label via ILAmbience.EscapeName.

Holding resolved entities means the tree is rebuilt when a setting changes the type system. Only one compilation is cached per module, keyed on the effective decompiler settings, so a language-version or decompiler-option change drops it and would otherwise leave every node pointing at a discarded compilation — stale labels, icons and filters, and the C# 14 extension-block nodes shown against the wrong version. AssemblyTreeModel reloads the loaded assemblies when the computed TypeSystemOptions actually change (Display-only settings never do, and cost nothing), then restores the selected node from its path — which re-expands its ancestors — the way Refresh does. The WPF host got this for free: its modal Options dialog rebuilt the tree on close, where the Avalonia page applies live.

Two deliberate departures from a literal 10.1 copy: keep the global-namespace - node, and keep the cached IsPublicAPI getter. Both index dictionaries are cleared on rebuild so a nested/flat toggle leaves no stale entries.

Tests

New NestedNamespaceTreeTests / NamespaceTreeNodeTests / TypeSystemStalenessTests cover: intermediate namespaces stay visible; nested-namespace and nested-type lookups resolve; reference navigation resolves in nested mode; a namespace's types and sub-namespaces interleave alphabetically; a language-version change re-resolves the tree against the new type system and restores the selected node; label escaping; and — as a guard — that a namespace whose types are all filtered out by ShowApiLevel is still hidden by the cascade (mutation-verified against the Filter => Match opt-out). Full ILSpy.Tests suite green.

Relationship to #3879

This overlaps #3879, which fixes the same display bug by keeping the lazy design and opting namespace nodes out of the filter cascade. This PR takes the alternative the discussion there converged on — restoring the WPF eager build — which fixes the display, the lag, and nested-mode navigation together, and keeps the cascade intact. #3879's own improvements (the Decompile fullName fix and the Name → FullName fixes in the MSDN/scope-search entries) are not included here and would be reintegrated on top.

[Copilot]

Pull request overview

This PR restores the WPF host’s eager namespace-band construction for the Avalonia assembly tree to fix missing namespaces and reduce first-expand latency when Use nested namespace structure is enabled. It also adds indexing to make namespace/type lookups O(1) at any nesting depth and updates navigation to use those lookups.

Changes:

• Eagerly builds the full namespace tree in AssemblyTreeNode in a single pass over top-level metadata types, with indexes for namespace and type lookups.
• Simplifies NamespaceTreeNode to a label holder (with display escaping) and removes per-namespace lazy child loading.
• Updates type navigation in TreeNodeLocator and adds Avalonia headless tests covering nested namespace visibility and lookup/navigation behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
ILSpy/TreeNodes/NamespaceTreeNode.cs	Removes per-namespace lazy loading, escapes display label, and relies on AssemblyTreeNode for populating children.
ILSpy/TreeNodes/AssemblyTreeNode.cs	Eagerly constructs namespace/type nodes in one pass and adds O(1) lookup dictionaries for namespaces and top-level types.
ILSpy/AssemblyTree/TreeNodeLocator.cs	Switches type reference resolution to use AssemblyTreeNode.FindTypeNode for nested-namespace correctness.
ILSpy.Tests/AssemblyList/NestedNamespaceTreeTests.cs	Adds coverage for nested-namespace visibility and lookup/navigation behavior in Avalonia headless tests.
ILSpy.Tests/AssemblyList/NamespaceTreeNodeTests.cs	Adds coverage for namespace label escaping and global-namespace rendering.

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