[mika76]

UndoStack.MarkAsOriginalFile() doesn't stop the next StartContinuedUndoGroup() from joining the undo group before it. So changes made after a save merge into the saved undo step:

• IsOriginalFile stays true although the document was changed;
• one undo goes back past the saved state.

var doc = new TextDocument();
doc.Insert(0, "a");
doc.UndoStack.MarkAsOriginalFile();      // saved: "a"
doc.UndoStack.StartContinuedUndoGroup();
doc.Insert(1, "b");
doc.UndoStack.EndUndoGroup();
// doc.UndoStack.IsOriginalFile is true, the text is "ab"
doc.UndoStack.Undo();
// the text is "", not the saved "a"

The fix: MarkAsOriginalFile stops continuation, as Undo, Redo and ClearAll already do. Groups after the save still continue each other.

Tests in UndoStackTests: not original after continuing past a save, undo returns to the saved state, and later groups still continue.