This incident not withstanding, persistent undo breaks my mental model of file state. I treat writes as commits, which allows an open and modify session to be an exploratory branch until I decide to save it.

It is counterintuitive in the same manner as when excel persists undo history chronologically across the running application process, and not separately per document.

Similarly AutoCAD intersperses application parameters and tool configurations alongside actual data operations in it's undo history.