Stop treating position -1 as clean after trimming or saving

UndoRedo.Test/UndoRedoStackTests.cs

@@ -273,6 +273,71 @@ public void Execute_MaxStackSizeReached_RemovesOldestCommands()
 		Assert.AreEqual("Command 5", stack.Commands[2].Description); // Newest command
 	}
 
+	[TestMethod]
+	public void HasUnsavedChanges_AtStartAfterTrimming_IsTrue()
+	{
+		// Arrange: B trims A, so position -1 now holds A's never-saved result
+		UndoRedoService stack = new(new StackManager(), new SaveBoundaryManager(), new CommandMerger(), UndoRedoOptions.Create(maxStackSize: 1));
+		int value = 0;
+
+		stack.Execute(new DelegateCommand("A", () => value = 1, () => value = 0));
+		stack.Execute(new DelegateCommand("B", () => value = 2, () => value = 1));
+
+		// Act
+		stack.Undo();
+
+		// Assert
+		Assert.AreEqual(-1, stack.CurrentPosition);
+		Assert.AreEqual(1, value);
+		Assert.IsTrue(stack.HasUnsavedChanges, "Position -1 is not the initial state once trimming has shifted A's result there");
+	}
+
+	[TestMethod]
+	public void HasUnsavedChanges_AtStartAfterBranchRemovesTheOnlyBoundary_IsTrue()
+	{
+		// Arrange: A is saved, then undone and branched away from
+		UndoRedoService stack = CreateService();
+		int value = 0;
+
+		stack.Execute(new DelegateCommand("A", () => value = 1, () => value = 0));
+		stack.MarkAsSaved();
+		stack.Undo();
+		Assert.IsTrue(stack.HasUnsavedChanges, "The saved file holds A, not the initial state");
+
+		stack.Execute(new DelegateCommand("B", () => value = 2, () => value = 0));
+		Assert.IsEmpty(stack.SaveBoundaries, "The branch invalidates the boundary at A");
+
+		// Act
+		stack.Undo();
+
+		// Assert: the file on disk still holds A
+		Assert.AreEqual(-1, stack.CurrentPosition);
+		Assert.AreEqual(0, value);
+		Assert.IsTrue(stack.HasUnsavedChanges, "The initial state was never what was saved");
+	}
+
+	[TestMethod]
+	public void AdjustPositions_BoundaryShiftedToStart_IsKept()
+	{
+		// Arrange
+		UndoRedoService stack = new(new StackManager(), new SaveBoundaryManager(), new CommandMerger(), UndoRedoOptions.Create(maxStackSize: 2));
+
+		stack.Execute(new DelegateCommand("A", () => { }, () => { }));
+		stack.MarkAsSaved("after A");
+		stack.Execute(new DelegateCommand("B", () => { }, () => { }));
+
+		// Act: C trims A, which shifts the save point after A to -1
+		stack.Execute(new DelegateCommand("C", () => { }, () => { }));
+
+		// Assert
+		Assert.HasCount(1, stack.SaveBoundaries, "A save point shifted to -1 is still reachable and must be kept");
+		Assert.AreEqual(-1, stack.SaveBoundaries[0].Position);
+
+		stack.Undo();
+		stack.Undo();
+		Assert.IsFalse(stack.HasUnsavedChanges, "Undoing back to the save point after A must report it as saved");
+	}
+
 	[TestMethod]
 	public void CommandMerging_ConsecutiveCommands_MergesCorrectly()
 	{

UndoRedo/Services/SaveBoundaryManager.cs

@@ -11,16 +11,20 @@ public sealed class SaveBoundaryManager : ISaveBoundaryManager
 {
 	private readonly List<SaveBoundary> _saveBoundaries = [];
 
+	// Whether position -1 still holds the untouched initial state, which is clean without a boundary.
+	// It stops being true once anything is saved, since the saved state replaces it, and once trimming
+	// shifts later commands' results down to -1.
+	private bool _initialStateIsClean = true;
+
 	/// <inheritdoc />
 	public IReadOnlyList<SaveBoundary> SaveBoundaries => _saveBoundaries.AsReadOnly();
 
 	/// <inheritdoc />
 	public bool HasUnsavedChanges(int currentPosition)
 	{
-		// If no save boundaries exist, we have unsaved changes unless at initial position
-		if (_saveBoundaries.Count == 0)
+		if (currentPosition == -1 && _initialStateIsClean)
 		{
-			return currentPosition >= 0;
+			return false;
 		}
 
 		// No unsaved changes if we're exactly at a save boundary position
@@ -32,6 +36,7 @@ public SaveBoundary CreateSaveBoundary(int position, string? description = null)
 	{
 		SaveBoundary saveBoundary = new(position, description);
 		_saveBoundaries.Add(saveBoundary);
+		_initialStateIsClean = false;
 		return saveBoundary;
 	}
 
@@ -58,12 +63,19 @@ public void AdjustPositions(int adjustment)
 			return;
 		}
 
+		if (adjustment < 0)
+		{
+			// Commands were trimmed from the bottom, so -1 is now the state after them, not the initial one
+			_initialStateIsClean = false;
+		}
+
 		for (int i = _saveBoundaries.Count - 1; i >= 0; i--)
 		{
 			SaveBoundary boundary = _saveBoundaries[i];
 			int newPosition = boundary.Position + adjustment;
 
-			if (newPosition < 0)
+			// -1 is a reachable position, so a boundary shifted exactly there is still a valid save point
+			if (newPosition < -1)
 			{
 				_saveBoundaries.RemoveAt(i);
 			}
@@ -90,5 +102,9 @@ public IEnumerable<ICommand> GetCommandsToUndo(SaveBoundary saveBoundary, int cu
 	}
 
 	/// <inheritdoc />
-	public void Clear() => _saveBoundaries.Clear();
+	public void Clear()
+	{
+		_saveBoundaries.Clear();
+		_initialStateIsClean = true;
+	}
 }