`_file_identity` keyed on normcase(abspath), which folds case and separators
only, and only on Windows. Two names for one file — a hard link (which
Path.resolve() cannot fold either), an NTFS junction reached in-process, or
a case variant on macOS's case-insensitive APFS where normcase is the
identity — registered two entries: both rows said `saved: True`, a save from
either overwrote the other's bytes, and closing the 'clean' one dropped its
work as `discarded_changes: False`.
Identity now asks the disk: `os.stat` (st_dev, st_ino) when the path exists
(the samefile identity), `normcase(abspath)` only for a path not on disk yet
(a save target, or an owner whose file was removed) or a filesystem that
reports no inode. `_register_document`, `_path_holder`,
`_resolve_document_key`, the `_doc_path` setter and the quarantine's
same-path warning all go through the one helper. An untitled entry holds no
file and never matches a path.
Tests: the `_case_folding_fs` guard probes the temp filesystem instead of
`os.path.normcase`, so the case-variant tests run on macOS where they used
to skip while the bug was live. New: hard link (backend and over the wire),
junction (mklink /J; a directory symlink elsewhere), a case variant with
normcase given its POSIX definition, and the spelling fallback for a save
target that is not on disk. Four of the five fail on the previous backend.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
