Clicking a version in Version history shows it in place of the document,
read-only, with a banner to restore it or go back to the current version
(clicking Current version or closing the panel goes back too). Edits,
undo and gestures are refused while a version is shown; saving, closing
and restoring end the preview first, so the document and its unsaved
state are never touched by it.
New commands version.preview and version.exitPreview; version.list marks
the shown version.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
