85f79283 made _calculate_folder compare root identities, but sliced the
relative folder out of the identity itself -- and normalize_root_identity
folds case on Windows (or any drive-letter path), so every move recorded
the folder in lowercase. The sidebar then grew a lowercase twin branch
("Krea 2" vs "krea 2") on each move until the next disk walk healed it.
Slice from normalize_root_spelling instead, a case-preserving counterpart
that folds only separators and redundancies, with a length guard for the
rare Unicode case-fold that changes the string length.
Will Miao
cf3877bbc9