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.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Mzze7shr5pZoEgKQU5NW

tests/test_round39.py

@@ -0,0 +1,247 @@
+"""Round 39: playlist merges stop losing your track order.
+
+Three defects, and it took all three to scramble `a nissa one`:
+
+* `_merge_playlist` was a 2-way union with no common ancestor — it took one
+  side's order wholesale and *appended* the other side's extras, so a track
+  inserted in the middle on one machine arrived at the bottom on the other.
+* Whose order won was decided by the file's mtime, and the last column is
+  stretch-sized — so resizing the window rewrote the open playlist's JSON and
+  "most recently edited" meant "most recently resized".
+* Those cosmetic rewrites were themselves handing Syncthing conflicts.
+"""
+import json
+import random
+
+import pytest
+
+from lintunes.models import Library, Playlist, PlaylistType
+from lintunes.storage import json_storage, conflict_resolver
+from lintunes.storage.conflict_resolver import merge_track_order
+
+
+def _save(tmp_path, playlists=()):
+    lib = Library()
+    for p in playlists:
+        lib.playlists[p.persistent_id] = p
+    json_storage.save_library(lib, tmp_path)
+    return lib
+
+
+def _conflict_name(original: str) -> str:
+    stem, _, ext = original.rpartition(".")
+    return f"{stem}.sync-conflict-20240101-120000-ABCDEFG.{ext}"
+
+
+class TestAnchoredOrder:
+    """merge_track_order: a track inserted in the middle stays in the middle."""
+
+    def test_middle_insert_is_not_appended(self):
+        # The other copy slipped 99 between 2 and 3. It belongs there, not last.
+        assert merge_track_order([1, 2, 3, 4], [1, 2, 99, 3, 4]) == [1, 2, 99, 3, 4]
+
+    def test_both_sides_inserted_at_different_points(self):
+        assert merge_track_order([1, 7, 2, 3], [1, 2, 8, 3]) == [1, 7, 2, 8, 3]
+
+    def test_consecutive_inserts_keep_their_relative_order(self):
+        assert merge_track_order([1, 2], [1, 8, 9, 2]) == [1, 8, 9, 2]
+
+    def test_a_leading_insert_goes_to_the_head(self):
+        assert merge_track_order([1, 2, 3], [7, 1, 2, 3]) == [7, 1, 2, 3]
+
+    def test_primary_decides_the_order_of_shared_tracks(self):
+        # Both copies hold 1-2-3; primary reversed them. Primary wins.
+        assert merge_track_order([3, 2, 1], [1, 2, 3]) == [3, 2, 1]
+
+    def test_trailing_insert_still_lands_after_its_anchor(self):
+        assert merge_track_order([1, 2, 3], [1, 2, 3, 4]) == [1, 2, 3, 4]
+
+    @pytest.mark.parametrize("primary,secondary", [
+        ([], []),
+        ([1, 2, 3], []),
+        ([], [1, 2, 3]),
+        ([1, 2, 3], [4, 5, 6]),          # disjoint
+        ([1, 2, 2, 3], [1, 2, 3, 3]),    # the same track twice
+    ])
+    def test_edge_cases_never_drop_a_track(self, primary, secondary):
+        merged = merge_track_order(primary, secondary)
+        assert set(merged) == set(primary) | set(secondary)
+
+    def test_nothing_is_ever_dropped(self):
+        """The invariant that matters most: a merge is a union, always."""
+        rng = random.Random(39)
+        for _ in range(300):
+            base = rng.sample(range(20), rng.randint(0, 10))
+            a = [t for t in base if rng.random() > 0.3] + rng.sample(range(20, 30), 2)
+            b = [t for t in base if rng.random() > 0.3] + rng.sample(range(30, 40), 2)
+            rng.shuffle(a)
+            merged = merge_track_order(a, b)
+            assert set(merged) == set(a) | set(b)
+            # Primary's entries keep primary's exact sequence — only tracks it
+            # never had get woven in between them.
+            assert [t for t in merged if t in set(a)] == a
+
+
+class TestHonestDateModified:
+    """The merge decides on the playlist's own date_modified, not file mtime."""
+
+    def _setup(self, tmp_path, ours, theirs, our_stamp, their_stamp):
+        pl = Playlist(name="a nissa one", persistent_id="AAAA1111",
+                      playlist_type=PlaylistType.REGULAR, track_ids=ours,
+                      date_modified=our_stamp)
+        _save(tmp_path, [pl])
+        pfile = tmp_path / "playlists" / "AAAA1111.json"
+        other = json.loads(pfile.read_text())
+        other["track_ids"] = theirs
+        if their_stamp is None:
+            other.pop("date_modified", None)
+        else:
+            other["date_modified"] = their_stamp
+        cfile = tmp_path / "playlists" / _conflict_name("AAAA1111.json")
+        cfile.write_text(json.dumps(other))
+        return pfile, cfile
+
+    def test_a_resize_no_longer_beats_a_reorder(self, tmp_path):
+        """The `a nissa one` regression. We reordered; the other machine only
+        opened it and resized the window, so its *file* is newer — but its
+        date_modified is older, and our order must survive."""
+        pfile, cfile = self._setup(
+            tmp_path,
+            ours=[3, 1, 2],           # we carefully reordered
+            theirs=[1, 2, 3],         # they never touched the contents
+            our_stamp="2026-08-20T18:00:00",
+            their_stamp="2026-08-19T09:00:00")
+        import os, time
+        os.utime(cfile, (time.time() + 100, time.time() + 100))  # their file is newer
+
+        summaries = conflict_resolver.resolve_conflicts(tmp_path)
+
+        assert json.loads(pfile.read_text())["track_ids"] == [3, 1, 2]
+        assert "this machine" in summaries[0].lines[0]
+
+    def test_the_genuinely_newer_edit_still_wins(self, tmp_path):
+        pfile, _ = self._setup(
+            tmp_path, ours=[1, 2, 3], theirs=[3, 2, 1],
+            our_stamp="2026-08-19T09:00:00",
+            their_stamp="2026-08-20T18:00:00")
+
+        summaries = conflict_resolver.resolve_conflicts(tmp_path)
+
+        assert json.loads(pfile.read_text())["track_ids"] == [3, 2, 1]
+        assert "the other machine" in summaries[0].lines[0]
+
+    def test_their_insert_lands_in_position_not_at_the_tail(self, tmp_path):
+        pfile, _ = self._setup(
+            tmp_path, ours=[1, 2, 3], theirs=[1, 2, 99, 3],
+            our_stamp="2026-08-20T18:00:00",
+            their_stamp="2026-08-19T09:00:00")
+
+        summaries = conflict_resolver.resolve_conflicts(tmp_path)
+
+        assert json.loads(pfile.read_text())["track_ids"] == [1, 2, 99, 3]
+        assert "back in position" in summaries[0].lines[1]
+
+    def test_falls_back_to_mtime_before_the_field_existed(self, tmp_path):
+        """Playlists written by an older version carry no stamp."""
+        pfile, cfile = self._setup(
+            tmp_path, ours=[1, 2, 3], theirs=[3, 2, 1],
+            our_stamp=None, their_stamp=None)
+        import os, time
+        os.utime(cfile, (time.time() + 100, time.time() + 100))
+
+        conflict_resolver.resolve_conflicts(tmp_path)
+        assert json.loads(pfile.read_text())["track_ids"] == [3, 2, 1]
+
+    def test_smart_playlist_rules_also_use_date_modified(self, tmp_path):
+        from lintunes.smart import SmartCriteria
+        pl = Playlist(name="Recent", persistent_id="BBBB2222",
+                      playlist_type=PlaylistType.SMART, track_ids=[1],
+                      smart_criteria=SmartCriteria(), date_modified="2026-08-20T18:00:00")
+        _save(tmp_path, [pl])
+        pfile = tmp_path / "playlists" / "BBBB2222.json"
+        ours = json.loads(pfile.read_text())
+        other = dict(ours)
+        other["name"] = "Stale"
+        other["date_modified"] = "2026-08-19T09:00:00"
+        cfile = tmp_path / "playlists" / _conflict_name("BBBB2222.json")
+        cfile.write_text(json.dumps(other))
+        import os, time
+        os.utime(cfile, (time.time() + 100, time.time() + 100))
+
+        conflict_resolver.resolve_conflicts(tmp_path)
+        assert json.loads(pfile.read_text())["name"] == "Recent"
+
+
+class TestDateModifiedMovesOnlyForContent:
+    @pytest.fixture
+    def manager(self, tmp_path, qapp, monkeypatch):
+        monkeypatch.setenv("XDG_CONFIG_HOME", str(tmp_path / "config"))
+        from lintunes.library_manager import LibraryManager
+        from lintunes.models import Track
+        lib = Library()
+        for tid in (1, 2, 3):
+            lib.tracks[tid] = Track(track_id=tid, name=f"T{tid}")
+        lib.playlists["AAAA1111"] = Playlist(
+            name="a nissa one", persistent_id="AAAA1111",
+            playlist_type=PlaylistType.REGULAR, track_ids=[1, 2, 3])
+        return LibraryManager(lib, tmp_path)
+
+    def _stamp(self, manager):
+        return manager.library.playlists["AAAA1111"].date_modified
+
+    def test_reorder_bumps_it(self, manager):
+        assert self._stamp(manager) is None
+        manager.move_tracks_in_playlist("AAAA1111", [2], 0)
+        assert self._stamp(manager) is not None
+
+    def test_add_and_remove_bump_it(self, manager):
+        manager.remove_tracks_from_playlist("AAAA1111", [0])
+        first = self._stamp(manager)
+        assert first is not None
+        manager.add_tracks_to_playlist("AAAA1111", [1])
+        assert self._stamp(manager) >= first
+
+    def test_undo_bumps_it_too(self, manager):
+        manager.move_tracks_in_playlist("AAAA1111", [2], 0)
+        before = self._stamp(manager)
+        manager.undo_stack.undo()
+        assert manager.library.playlists["AAAA1111"].track_ids == [1, 2, 3]
+        assert self._stamp(manager) >= before
+
+    def test_a_column_resize_does_not(self, manager):
+        """The whole point: cosmetic settings must not look like an edit."""
+        manager.mark_playlist_settings_dirty("AAAA1111")
+        assert self._stamp(manager) is None
+
+
+class TestStretchedColumnIsNotPersisted:
+    @pytest.fixture
+    def table(self, qapp):
+        from lintunes.gui.track_table import TrackTableView
+        from lintunes.models import Track
+        view = TrackTableView()
+        view.set_tracks([Track(track_id=1, name="A", artist="B")])
+        return view
+
+    def test_a_middle_column_still_persists(self, table):
+        seen = []
+        table.column_width_changed.connect(lambda f, w: seen.append((f, w)))
+        table.horizontalHeader().resizeSection(1, 123)
+        assert seen and seen[0][1] == 123
+
+    def test_the_stretched_last_column_does_not(self, table):
+        """Qt re-fires sectionResized for the stretched column on every viewport
+        width change — a window resize used to rewrite the playlist JSON."""
+        header = table.horizontalHeader()
+        last = header.logicalIndex(header.count() - 1)
+        seen = []
+        table.column_width_changed.connect(lambda f, w: seen.append((f, w)))
+        header.resizeSection(last, 321)
+        assert seen == []
+
+    def test_resizing_the_widget_is_silent(self, table):
+        seen = []
+        table.column_width_changed.connect(lambda f, w: seen.append((f, w)))
+        table.resize(400, 300)
+        table.resize(900, 300)
+        assert seen == []