BlockMap::sync unwrapped the transform cursor for every edit, assuming
each edit's old.start lands strictly inside the old transform tree. The
companion (split-diff) branch of sync can compose an edit anchored at
the trailing boundary of the old transforms, leaving the cursor past the
end of the tree and aborting the process on the None unwrap. Only bind
the transform when there are rows preceding the edit.
Fixes ZED-9V4.
Closes FR-113
Release Notes:
- N/A or Added/Fixed/Improved ...
Lukas Wirth
5460d079cf