v0.9.1: playlist merges stop losing your track order

What scrambled `a nissa one` took three defects at once. Reorder it on machine
A; open it on machine B and resize the window; B's file is now newer, so the
merge takes B's order — the old one — wholesale.

_merge_playlist was a 2-way union with no common ancestor: one side's order
wholesale, the other side's extras appended at the tail, so a track inserted in
the middle on one machine arrived at the bottom on the other. merge_track_order
re-inserts each side-only track after the nearest track both copies share
instead. _reconcile_playlist (the live-reload path) uses the same helper. The
union invariant is unchanged and property-tested over 300 random pairs — a
merge never drops a track.

Whose order wins is now Playlist.date_modified, bumped only in _set_track_ids
(add / remove / reorder / undo) and deliberately not by
mark_playlist_settings_dirty, with a file-mtime fallback for playlists written
before the field existed. The file's mtime was a lie: the last column is
stretch-sized, so Qt re-fires sectionResized whenever the viewport width
changes, and a window resize or splitter drag rewrote the open playlist's JSON
— moving its mtime and handing Syncthing another conflict — for a width
apply_settings overrides on load anyway. _on_section_resized now skips that
section; genuine drags on every other column still persist.

Replayed on a copy of the real playlists: `a nissa one` keeps its reorder
against a newer-by-mtime opponent, and `a nissa ideas` takes the other
machine's two mid-list inserts at 5 and 12 rather than 26 and 27.

Claude-Session: https://claude.ai/code/session_01M8Mzze7shr5pZoEgKQU5NW

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.9.0"
+__version__ = "0.9.1"

lintunes/gui/track_table.py

@@ -714,6 +714,15 @@ class TrackTableView(QTableView):
     def _on_section_resized(self, section, _old, new_width):
         if self._suppress_signals or section >= len(self.model_.fields):
             return
+        header = self.horizontalHeader()
+        if (header.stretchLastSection()
+                and section == header.logicalIndex(header.count() - 1)):
+            # The last column is stretch-sized, so Qt re-fires this every time
+            # the viewport changes width — resizing the window or dragging the
+            # splitter would otherwise rewrite the whole playlist JSON (and hand
+            # Syncthing a conflict). Its width is derived, not chosen: there's no
+            # right edge to drag, and apply_settings overrides it on load.
+            return
         field = self.model_.fields[section]
         if field != INDEX_FIELD:
             self.column_width_changed.emit(field, new_width)

lintunes/library_manager.py

@@ -313,6 +313,11 @@ class LibraryManager(QObject):
         if playlist is None:
             return
         playlist.track_ids = list(ids)
+        # The single funnel for every content change (add / remove / reorder /
+        # undo), and deliberately the *only* place date_modified moves — a
+        # column resize must not look like an edit to the merge. See
+        # conflict_resolver._playlist_newer.
+        playlist.date_modified = _utc_now_iso()
         self._mark_playlist(pid)
         self.playlist_content_changed.emit(pid)
 
@@ -974,13 +979,13 @@ def _reconcile_track(mem_track, disk_track):
 
 
 def _reconcile_playlist(mem_pl, disk_pl):
-    """Reconcile a playlist we were locally editing with the disk copy: keep our
-    order and append any tracks that exist only on the other machine (a merge
+    """Reconcile a playlist we were locally editing with the disk copy: our order
+    wins (the local edit is the more recent one), and tracks that exist only on
+    the other machine are merged back into position rather than appended (a merge
     never drops tracks)."""
-    local = list(mem_pl.track_ids)
-    local_set = set(local)
-    mem_pl.track_ids = local + [tid for tid in disk_pl.track_ids
-                                if tid not in local_set]
+    from lintunes.storage.conflict_resolver import merge_track_order
+    mem_pl.track_ids = merge_track_order(list(mem_pl.track_ids),
+                                         list(disk_pl.track_ids))
 
 
 def _utc_now_iso() -> str:

lintunes/models/playlist.py

@@ -58,6 +58,10 @@ class Playlist:
     track_ids: list[int] = field(default_factory=list)
     settings: PlaylistSettings = field(default_factory=PlaylistSettings)
     is_system: bool = False
+    # Bumped only when track_ids change (LibraryManager._set_track_ids), never
+    # for column widths or sort order — the conflict merge uses it to decide
+    # whose order wins, and the file's mtime moves for cosmetic reasons.
+    date_modified: Optional[str] = None
     # Set only for SMART playlists. Membership (track_ids) is derived from this
     # by the LibraryManager; for imported playlists track_ids also doubles as a
     # snapshot fallback when the criteria couldn't be fully parsed.
@@ -78,6 +82,8 @@ class Playlist:
         }
         if self.parent_persistent_id:
             d["parent_persistent_id"] = self.parent_persistent_id
+        if self.date_modified:
+            d["date_modified"] = self.date_modified
         if self.smart_criteria is not None:
             d["smart_criteria"] = self.smart_criteria.to_dict()
         return d
@@ -96,6 +102,7 @@ class Playlist:
             track_ids=d.get("track_ids", []),
             settings=PlaylistSettings.from_dict(d.get("settings", {})),
             is_system=d.get("is_system", False),
+            date_modified=d.get("date_modified"),
             smart_criteria=smart_criteria,
         )
 

lintunes/storage/conflict_resolver.py

@@ -226,43 +226,102 @@ def _merge_track_fields(orig: dict, conflict: dict) -> list[str]:
     return notes
 
 
+def merge_track_order(primary: list[int], secondary: list[int]) -> list[int]:
+    """Union two orderings of a playlist without losing position.
+
+    A plain union appends whatever the other copy had at the tail, so a track
+    inserted in the *middle* on one machine arrives at the *bottom* on the other
+    — which is what scrambled `a nissa one`. Instead, every track that exists
+    only in ``secondary`` is re-inserted after its nearest preceding track that
+    both copies share (its "anchor"); ``primary`` decides the order of everything
+    the two have in common.
+
+    The result is always the union of the two lists: a merge must never drop a
+    track, so anything whose anchor never turns up is flushed at the tail.
+    """
+    common = set(primary) & set(secondary)
+
+    # Walk secondary once: each secondary-only track hangs off the last shared
+    # track before it (None = it led the list).
+    pending: dict[object, list[int]] = {}
+    anchor = None
+    for tid in secondary:
+        if tid in common:
+            anchor = tid
+        else:
+            pending.setdefault(anchor, []).append(tid)
+
+    merged = list(pending.pop(None, []))
+    seen = set()
+    for tid in primary:
+        merged.append(tid)
+        # First occurrence only — a playlist may hold the same track twice.
+        if tid in common and tid not in seen:
+            seen.add(tid)
+            merged.extend(pending.pop(tid, []))
+    for leftover in pending.values():  # anchor never appeared; never drop it
+        merged.extend(leftover)
+    return merged
+
+
+def _playlist_newer(original: dict, conflict: dict,
+                    original_path: Path, conflict_path: Path) -> bool:
+    """Is the incoming copy the more recently *edited* one?
+
+    Prefer the playlist's own ``date_modified``, which only moves when its
+    contents change. File mtime is a lie: the last column is stretch-sized, so
+    resizing the window rewrites the open playlist's JSON — "most recently
+    edited" used to mean "most recently resized". Falls back to mtime for
+    playlists written before ``date_modified`` existed.
+    """
+    ours, theirs = original.get("date_modified"), conflict.get("date_modified")
+    if ours and theirs:
+        return theirs > ours
+    return conflict_path.stat().st_mtime > original_path.stat().st_mtime
+
+
 def _merge_playlist(original_path: Path, conflict_path: Path) -> ConflictSummary:
     original = read_json(original_path)
     conflict = read_json(conflict_path)
     name = original.get("name") or conflict.get("name") or original_path.stem
 
+    conflict_newer = _playlist_newer(original, conflict, original_path, conflict_path)
+
     # Smart playlists: membership is derived, so keep the newer criteria and let
     # it recompute (unioning derived track_ids would resurrect non-matches).
     if original.get("playlist_type") == "smart" or conflict.get("playlist_type") == "smart":
-        newer = _keep_newer(original_path, conflict_path)
-        newer.file, newer.kind = name, "playlist"
-        newer.lines = ["Smart-playlist rules taken from the most recently edited copy."]
-        return newer
+        if conflict_newer:
+            write_json(original_path, conflict)
+        return ConflictSummary(
+            name, "playlist",
+            ["Smart-playlist rules taken from the most recently edited copy."])
 
     orig_ids = list(original.get("track_ids", []))
     conf_ids = list(conflict.get("track_ids", []))
     orig_set, conf_set = set(orig_ids), set(conf_ids)
 
-    conflict_newer = conflict_path.stat().st_mtime > original_path.stat().st_mtime
     if conflict_newer:
-        base_order = conf_ids
-        extra = [tid for tid in orig_ids if tid not in conf_set]
-        original["track_ids"] = base_order + extra
+        # The other copy was edited last, so it sets the order; anything only we
+        # had is re-inserted at its anchor rather than dumped at the tail.
+        original["track_ids"] = merge_track_order(conf_ids, orig_ids)
         if "settings" in conflict:
             original["settings"] = conflict["settings"]
+        if conflict.get("date_modified"):
+            original["date_modified"] = conflict["date_modified"]
         order_src = "the other machine"
+        moved_in = len(conf_set - orig_set)
     else:
-        extra = [tid for tid in conf_ids if tid not in orig_set]
-        original["track_ids"] = orig_ids + extra
+        original["track_ids"] = merge_track_order(orig_ids, conf_ids)
         order_src = "this machine"
+        moved_in = len(orig_set - conf_set)
 
     write_json(original_path, original)
 
     added = len(set(original["track_ids"]) - orig_set)
     lines = [f"Order kept from {order_src} (most recently edited)."]
     if added:
-        lines.append(f"{added} track(s) that were only in the other copy were kept "
-                     "(nothing is removed on a merge).")
+        lines.append(f"{added} track(s) that were only in the other copy were kept, "
+                     "back in position (nothing is removed on a merge).")
     return ConflictSummary(name, "playlist", lines)