Undo stack is cleared when programmatically modifying content

Phenomenon

When programmatically modifying the content of a contenteditable region (e.g., using innerHTML or textContent), the undo stack is cleared in Safari. This prevents users from undoing their previous manual edits.

Reproduction example

1. Create a contenteditable div.
2. Type some text manually.
3. Use JavaScript to modify the content: element.innerHTML = 'New content'.
4. Try to undo (Cmd+Z).
5. Observe whether the previous manual edits can be undone.

Observed behavior

• In Safari on macOS, programmatic content changes clear the undo stack.
• Users cannot undo their previous manual edits after programmatic changes.
• The undo history is lost unexpectedly.

Expected behavior

• Programmatic changes should not clear the undo stack for manual edits.
• Or, there should be a way to preserve undo history across programmatic modifications.
• The undo stack should be managed more intelligently.