Stop auto-merge folding an edit into a saved command

UndoRedo.Test/UndoRedoStackTests.cs

@@ -402,6 +402,55 @@ public void CommandMerging_AfterUndo_CleansInvalidSaveBoundaries()
 		Assert.IsFalse(stack.CanRedo, "Redo should be unavailable after forward commands are cleared");
 	}
 
+	[TestMethod]
+	public void CommandMerging_AfterMarkAsSaved_KeepsTheSaveBoundary()
+	{
+		// Arrange
+		UndoRedoOptions options = UndoRedoOptions.Create(autoMerge: true);
+		UndoRedoService stack = new(new StackManager(), new SaveBoundaryManager(), new CommandMerger(), options);
+		List<char> value = [];
+
+		stack.Execute(new TestInsertMergeCommand(value, 0, "a"));
+		stack.MarkAsSaved();
+
+		// Act
+		stack.Execute(new TestInsertMergeCommand(value, 1, "b"));
+
+		// Assert
+		Assert.AreEqual("ab", new string([.. value]));
+		Assert.AreEqual(2, stack.CommandCount, "An edit after a save should not merge into the saved command");
+		Assert.HasCount(1, stack.SaveBoundaries, "The save boundary should survive the next edit");
+		Assert.IsTrue(stack.HasUnsavedChanges, "The edit after the save should be unsaved");
+
+		stack.Undo();
+		Assert.AreEqual("a", new string([.. value]), "Undo should return to the saved text");
+		Assert.IsFalse(stack.HasUnsavedChanges, "Undo back to the save should leave the stack clean");
+	}
+
+	[TestMethod]
+	public void CommandMerging_AfterMarkAsSaved_MergesLaterEditsWithEachOther()
+	{
+		// Arrange
+		UndoRedoOptions options = UndoRedoOptions.Create(autoMerge: true);
+		UndoRedoService stack = new(new StackManager(), new SaveBoundaryManager(), new CommandMerger(), options);
+		List<char> value = [];
+
+		stack.Execute(new TestInsertMergeCommand(value, 0, "a"));
+		stack.MarkAsSaved();
+
+		// Act
+		stack.Execute(new TestInsertMergeCommand(value, 1, "b"));
+		stack.Execute(new TestInsertMergeCommand(value, 2, "c"));
+
+		// Assert
+		Assert.AreEqual("abc", new string([.. value]));
+		Assert.AreEqual(2, stack.CommandCount, "Edits with no save between them should still merge");
+
+		stack.Undo();
+		Assert.AreEqual("a", new string([.. value]), "Undo should revert the merged post-save edits together");
+		Assert.IsFalse(stack.HasUnsavedChanges);
+	}
+
 	[TestMethod]
 	public void CompositeCommand_NestedComposites_HandlesCorrectly()
 	{

UndoRedo/Services/UndoRedoService.cs

@@ -70,13 +70,17 @@
 	/// <param name="navigationProvider">The navigation provider</param>
 	public void SetNavigationProvider(INavigationProvider? navigationProvider) => _navigationProvider = navigationProvider;
 
+	private bool IsAtSaveBoundary(int position) => _saveBoundaryManager.SaveBoundaries.Any(boundary => boundary.Position == position);
+
 	/// <inheritdoc />
 	public void Execute(ICommand command)
 	{
 		Ensure.NotNull(command);
 
-		// Try to merge with the last command if auto-merge is enabled
-		if (_options.AutoMergeCommands && _stackManager.CanUndo)
+		// Try to merge with the last command if auto-merge is enabled. Never merge into a command that
+		// ends at a save boundary: that would fold the new edit into the saved state and delete the
+		// boundary, so Undo could never return to what was saved.
+		if (_options.AutoMergeCommands && _stackManager.CanUndo && !IsAtSaveBoundary(_stackManager.CurrentPosition))
 		{
 			ICommand? lastCommand = _stackManager.GetCurrentCommand();
 			if (lastCommand != null && _commandMerger.CanMerge(lastCommand, command))
@@ -168,7 +172,7 @@
 
 			try
 			{
 				await _navigationProvider.NavigateToAsync(command.NavigationContext!, cts.Token).ConfigureAwait(false);
 			}
 			catch (OperationCanceledException)
 			{
@@ -204,7 +208,7 @@
 
 			try
 			{
 				await _navigationProvider.NavigateToAsync(command.NavigationContext!, cts.Token).ConfigureAwait(false);
 			}
 			catch (OperationCanceledException)
 			{
@@ -270,7 +274,7 @@
 
 			try
 			{
 				await _navigationProvider.NavigateToAsync(lastCommand.NavigationContext!, cts.Token).ConfigureAwait(false);
 			}
 			catch (OperationCanceledException)
 			{