Speed up moving rows

Fixes #262
Fixed #260

app/playlistmodel.py

@@ -990,12 +990,28 @@ class PlaylistModel(QAbstractTableModel):
 
     @line_profiler.profile
     def refresh_data(self, session: db.session, dummy_for_profiling=None) -> None:
-        """Populate dicts for data calls"""
+        """Populate self.playlist_rows with playlist data"""
 
-        # Populate self.playlist_rows with playlist data
-        self.playlist_rows.clear()
+        # We used to clear self.playlist_rows each time but that's
+        # expensive and slow on big playlists
+
+        # Note where each playlist_id is
+        plid_to_row: dict[int, int] = {}
+        for oldrow in self.playlist_rows:
+            plrdata = self.playlist_rows[oldrow]
+            plid_to_row[plrdata.playlistrow_id] = plrdata.row_number
+
+        # build a new playlist_rows
+        new_playlist_rows: dict[int, RowAndTrack] = {}
         for p in PlaylistRows.get_playlist_rows(session, self.playlist_id):
-            self.playlist_rows[p.row_number] = RowAndTrack(p)
+            if p.id not in plid_to_row:
+                new_playlist_rows[p.row_number] = RowAndTrack(p)
+            else:
+                new_playlist_rows[p.row_number] = self.playlist_rows[plid_to_row[p.id]]
+                new_playlist_rows[p.row_number].row_number = p.row_number
+
+        # Copy to self.playlist_rows
+        self.playlist_rows = new_playlist_rows
 
     def refresh_row(self, session, row_number):
         """Populate dict for one row from database"""