NavigateTo copies the whole history on every call, even with no undo provider; with an undo provider each retained action keeps two full snapshots (O(n) per navigation, ~125 MB at 50k entries)

[matt-edmondson]

What's wrong

Navigation<T>.NavigateTo snapshots the whole item list on every call, even when no undo provider is configured and the snapshot is thrown away:

Navigation/Navigation/Services/NavigationStack.cs

Lines 43 to 69 in d4bd627

	public void NavigateTo(T item)
	{
	Ensure.NotNull(item);
	T? previousItem = Current;
	// Capture state before navigation for undo
	int beforeIndex = _currentIndex;
	List<T> beforeItems = [.. _items];
	// Remove any forward history when navigating to a new item
	if (_currentIndex < _items.Count - 1)
	{
	_items.RemoveRange(_currentIndex + 1, _items.Count - _currentIndex - 1);
	}
	_items.Add(item);
	_currentIndex = _items.Count - 1;
	// Create undoable action if undo/redo provider is available
	if (undoRedoProvider != null)
	{
	NavigateToAction<T> action = new(this, beforeIndex, beforeItems, _currentIndex, [.. _items]);
	undoRedoProvider.RegisterAction(action, $"Navigate to {item.DisplayName}");
	}
	OnNavigationChanged(NavigationType.NavigateTo, previousItem, Current);

With an undo provider it copies the list again, and NavigateToAction then copies both lists once more:

Navigation/Navigation/Services/NavigateToAction.cs

Lines 20 to 33 in d4bd627

	internal sealed class NavigateToAction<T>(Navigation<T> navigation, int beforeIndex, List<T> beforeItems, int afterIndex, List<T> afterItems) : IUndoableAction where T : class, INavigationItem
	{
	private readonly Navigation<T> _navigation = Ensure.NotNull(navigation);
	private readonly List<T> _beforeItems = [.. beforeItems];
	private readonly List<T> _afterItems = [.. afterItems];
	/// <inheritdoc />
	public string Description => afterItems.Count > 0 ? $"Navigate to {afterItems[afterIndex].DisplayName}" : "Navigate";
	/// <inheritdoc />
	public void Execute() => _navigation.RestoreState(_afterItems, afterIndex);
	/// <inheritdoc />
	public void Undo() => _navigation.RestoreState(_beforeItems, beforeIndex);

The result:

• Every navigation costs O(history).
• Each of the up to maxHistorySize retained undo actions holds two full copies of the history.
• The navigation list itself has no size cap.

This contradicts the docs:

• docs/Architecture.md promises "O(1) navigation operations" and says "Configurable history limits prevent memory leaks".
• docs/Design-Decisions.md ("History Limits … Prevents unbounded memory growth") doesn't hold, because the undo limit caps the number of snapshots, not their size.

Repro (net10.0, Release): time for 1000 further NavigateTo calls at a given history size

history=  1000 undo=False   8.0 ms  heap=0 MB
history=  1000 undo=True   18.9 ms  heap=3 MB
history= 10000 undo=False  31.9 ms  heap=2 MB
history= 10000 undo=True  648.5 ms  heap=24 MB
history= 50000 undo=False 167.7 ms  heap=10 MB
history= 50000 undo=True  677.0 ms  heap=125 MB

Suggested fix / acceptance criteria

• Take no snapshot when undoRedoProvider is null.
• Have NavigateToAction record only the delta instead of full before/after lists: the truncated forward items, the pushed item, and the before/after indices.
• Optionally add a maxHistorySize to Navigation<T> (or its factory) that drops the oldest entries, so the history itself is bounded as the docs claim.
• Tests:
  • Undo and redo after NavigateTo, including after forward history was truncated, restore identical stacks.
  • NavigateTo time stays flat as the history grows.
• Coordinate with Undo after GoBack, Clear or LoadStateAsync drops the newest page and jumps to an unrelated state #55, which changes what undo must restore after GoBack, Clear and Load.

[matt-edmondson]

Triage

• Category: Improvement (performance / memory; the documented "O(1) navigation" claim is currently false)
• Priority: Medium. Typical histories are small, but the cost grows to about 650 ms per 1000 navigations and 125 MB at 50k entries with an undo provider. The quick first step, skipping the snapshot when undoRedoProvider is null, is trivial.
• Area / suggested assignment: NavigateTo in Navigation/Services/NavigationStack.cs and NavigateToAction.cs. Either fix the code or correct docs/Architecture.md / docs/Design-Decisions.md.
• Duplicates: none. Coordinate with Undo after GoBack, Clear or LoadStateAsync drops the newest page and jumps to an unrelated state #55 as the issue notes, since both change what undo restores.
• In progress: no matching open PR.

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