Add drag auto-scroll near the list edges

While a track drag hovers within 28px of the track list's top/bottom
edge, the list scrolls that way (40ms timer) so the user can drop onto
off-screen rows. Qt's built-in autoscroll never fires because our
dragMoveEvent accepts the event itself, so this is driven manually; the
drop line refreshes as content scrolls under a stationary cursor.

lintunes/gui/track_table.py

@@ -6,7 +6,7 @@ from PyQt6.QtWidgets import (
 )
 from PyQt6.QtCore import (
     Qt, QAbstractTableModel, QModelIndex, QSortFilterProxyModel, pyqtSignal,
-    QMimeData, QUrl, QPoint, QPointF, QRectF, QMetaType,
+    QMimeData, QUrl, QPoint, QPointF, QRectF, QMetaType, QTimer,
 )
 from PyQt6.QtGui import (
     QDrag, QKeySequence, QShortcut, QPixmap, QPainter, QPen, QPolygonF, QColor,
@@ -23,6 +23,23 @@ from lintunes.gui import drag_ghost
 TRACKS_MIME = "application/x-lintunes-tracks"
 SORT_ROLE = Qt.ItemDataRole.UserRole
 
+# While dragging a track within this many pixels of the list's top/bottom edge,
+# the list auto-scrolls so off-screen rows can be reached.
+AUTOSCROLL_MARGIN = 28
+
+
+def autoscroll_direction(y: int, height: int, accepted: bool,
+                         margin: int = AUTOSCROLL_MARGIN) -> int:
+    """-1 to scroll up, +1 down, 0 to stop: based on how close the drag is to
+    the top/bottom edge. Only scrolls over a valid drop target (accepted)."""
+    if not accepted:
+        return 0
+    if y < margin:
+        return -1
+    if y > height - margin:
+        return 1
+    return 0
+
 # Manual play-order pseudo-column (playlist views only, always first)
 INDEX_FIELD = "#"
 
@@ -357,6 +374,14 @@ class TrackTableView(QTableView):
         self.setDropIndicatorShown(False)
         self._drop_indicator_y: int | None = None
 
+        # Edge auto-scroll during a drag (Qt's built-in autoscroll never runs
+        # because our dragMoveEvent accepts the event itself).
+        self._autoscroll_dir = 0
+        self._autoscroll_pos: QPoint | None = None
+        self._autoscroll_timer = QTimer(self)
+        self._autoscroll_timer.setInterval(40)
+        self._autoscroll_timer.timeout.connect(self._autoscroll_tick)
+
         QShortcut(QKeySequence.StandardKey.Copy, self,
                   context=Qt.ShortcutContext.WidgetShortcut,
                   activated=self.copy_selection)
@@ -679,20 +704,23 @@ class TrackTableView(QTableView):
             else:
                 # library view is not a drop target for tracks
                 accepted = self._playlist_mode
+        pos = event.position().toPoint()
         if accepted:
             event.acceptProposedAction()
-            self._set_drop_indicator(
-                self._indicator_y(event.position().toPoint()))
+            self._set_drop_indicator(self._indicator_y(pos))
         else:
             event.ignore()
             self._set_drop_indicator(None)
+        self._update_autoscroll(pos, accepted)
 
     def dragLeaveEvent(self, event):
         self._set_drop_indicator(None)
+        self._stop_autoscroll()
         super().dragLeaveEvent(event)
 
     def dropEvent(self, event):
         self._set_drop_indicator(None)
+        self._stop_autoscroll()
         mime = event.mimeData()
         drop_row = self._drop_row(event.position().toPoint())
         if mime.hasUrls():
@@ -751,6 +779,35 @@ class TrackTableView(QTableView):
             self._drop_indicator_y = y
             self.viewport().update()
 
+    # ---- drag auto-scroll ----
+
+    def _update_autoscroll(self, pos: QPoint, accepted: bool):
+        """Start/stop edge auto-scroll based on the drag's position."""
+        self._autoscroll_pos = pos
+        self._autoscroll_dir = autoscroll_direction(
+            pos.y(), self.viewport().height(), accepted)
+        if self._autoscroll_dir and not self._autoscroll_timer.isActive():
+            self._autoscroll_timer.start()
+        elif not self._autoscroll_dir and self._autoscroll_timer.isActive():
+            self._autoscroll_timer.stop()
+
+    def _autoscroll_tick(self):
+        bar = self.verticalScrollBar()
+        before = bar.value()
+        bar.setValue(before + self._autoscroll_dir)
+        if bar.value() == before:  # reached the top/bottom — nothing more to do
+            self._autoscroll_timer.stop()
+            return
+        # The cursor may be held still while content scrolls under it, so
+        # refresh the drop line for the rows now beneath it.
+        if self._autoscroll_pos is not None:
+            self._set_drop_indicator(self._indicator_y(self._autoscroll_pos))
+
+    def _stop_autoscroll(self):
+        self._autoscroll_dir = 0
+        self._autoscroll_pos = None
+        self._autoscroll_timer.stop()
+
     def paintEvent(self, event):
         super().paintEvent(event)
         if self._drop_indicator_y is None: