Firefox undo stack corrupted when DOM mutates during editing

In Firefox, programmatic DOM changes during typing (auto-formatting, spellcheck fixes, framework reconciliation) can desynchronize the internal undo stack. Undo/redo may jump to wrong snapshots or truncate history.

Problem Overview

Gecko’s contenteditable undo model ties to editing sessions and DOM mutations. When JavaScript or the browser inserts nodes between keystrokes, the undo ring may record boundaries that do not match user intent.

Observed Behavior

• Undo removes more or less text than the last visible edit.
• Redo becomes a no-op after certain mutations.
• Corruption worsens when combining IME composition with programmatic wraps.

Impact

Data loss perception, broken editor sync layers, and difficult-to-reproduce bug reports.

Browser Comparison

Chromium and WebKit have different undo granularity; this scenario is specifically about Firefox’s stack interaction with mid-input DOM changes.

Solutions

• Defer non-essential DOM mutations until compositionend or idle time.
• Batch structural changes; avoid wrapping runs on every keystroke.
• When you must mutate, consider replacing the undo stack with a custom document history (ProseMirror-style) for Firefox.

Best Practices

• Reproduce with Firefox ESR and current release; behavior evolves.

Related Cases

• ce-0224

References

• MDN: Document.execCommand (legacy, but relevant to undo stacks)
• Firefox bug tracker: search for contenteditable undo + mutation