exceptions thrown when undoing (SwiftUI, macOS)

[wtholliday]

To reproduce (macOS 11.6 (20G165)):

1. Run attached project
2. Create a new document
3. Place insertion point at the end of the document
4. Add two characters
5. Undo 2x
6. Observe: *** -[NSBigMutableString substringWithRange:]: Range {14, 1} out of bounds; string length 14
7. Redo.
8. Observe: __redoCommonDoSingle:: NSUndoManager 0x600003d12580 is in invalid state, do not invoke this method while undoing

UndoTest.zip

[wtholliday]

Given the stack trace, this seems like it might be an apple bug:

0   CoreFoundation                      0x00007fff205ab1db __exceptionPreprocess + 242
	1   libobjc.A.dylib                     0x00007fff202e4d92 objc_exception_throw + 48
	2   Foundation                          0x00007fff2128c91d -[NSString substringWithRange:] + 223
	3   AppKit                              0x00007fff22f66881 -[NSTextStorage(NSUndo) _undoRedoAttributedSubstringFromRange:] + 145
	4   AppKit                              0x00007fff23514fd7 -[NSUndoTyping undoRedo:] + 142
	5   Foundation                          0x00007fff21390e26 -[_NSUndoStack popAndInvoke] + 187
	6   Foundation                          0x00007fff21390b9d -[NSUndoManager undoNestedGroup] + 296
	7   AppKit                              0x00007fff22f592bb -[NSApplication(NSResponder) sendAction:to:from:] + 288
	8   AppKit                              0x00007fff2305bcc9 -[NSMenuItem _corePerformAction] + 413

[wtholliday]

This change works around the behavior: https://github.com/audulus/Sourceful/commit/b724bc8e235e72618b0c30df4a2f87af29ae91d4

Undo still works, but each undo will change a single character, instead of the typical smarter NSTextView behavior.

[twostraws]

Single character undo seems like a pretty serious breakage. Is there really no other approach here?

[wtholliday]

@twostraws I'm not sure. There's a somewhat similar bug with SwiftUI's TextEditor which I've filed with Apple. Once they fix that, the situation here might improve.