v0.1.6: highlight the folder a dragged playlist would drop into

lintunes/gui/sidebar.py

@@ -157,6 +157,9 @@ class PlaylistTree(QTreeWidget):
         super().__init__(parent)
         self._manager = manager
         self._drop_target: QTreeWidgetItem | None = None
+        # KIND_ROLE value a drop during the active drag would land on
+        # ("folder" for internal playlist moves, "playlist" for track drops).
+        self._drag_target_kind: str | None = None
 
         self.setHeaderHidden(True)
         self.setIndentation(14)
@@ -396,13 +399,14 @@ class PlaylistTree(QTreeWidget):
             # so edge auto-scroll stays live even over non-folder rows —
             # otherwise scrolling down to a distant folder would be nearly
             # impossible past the playlists in between.
-            self._clear_drop_target()
+            self._drag_target_kind = "folder"
             target = self.itemAt(pos)
             kind = target.data(0, KIND_ROLE) if target else ""
             if kind in ("folder", "") or target is None:
                 event.acceptProposedAction()
             else:
                 event.ignore()
+            self._highlight_drop_target(pos, "folder")
             self._update_autoscroll(pos, accepted=True)
             return
         if not event.mimeData().hasFormat(TRACKS_MIME):
@@ -410,23 +414,48 @@ class PlaylistTree(QTreeWidget):
             self._update_autoscroll(pos, accepted=False)
             event.ignore()
             return
+        self._drag_target_kind = "playlist"
         target = self.itemAt(pos)
         if target and target.data(0, KIND_ROLE) == "playlist":
-            self._set_drop_target(target)
             event.acceptProposedAction()
             self._update_autoscroll(pos, accepted=True)
         else:
-            self._clear_drop_target()
             event.ignore()
             self._update_autoscroll(pos, accepted=False)
+        self._highlight_drop_target(pos, "playlist")
 
     def dragLeaveEvent(self, event):
         self._clear_drop_target()
         self._stop_autoscroll()
         super().dragLeaveEvent(event)
 
+    def _highlight_drop_target(self, pos: QPoint, kind: str):
+        """Highlight the row at ``pos`` if it's a valid target for the active
+        drag — a folder for internal playlist moves, a playlist for track
+        drops — so the user can see where the drop would land. Clears any
+        stale highlight otherwise. A folder inside the dragged row's own
+        branch is never highlighted: the manager would reject that drop as a
+        cycle, so lighting it up would lie."""
+        target = self.itemAt(pos)
+        ok = target is not None and target.data(0, KIND_ROLE) == kind
+        if ok and kind == "folder":
+            ok = not self._is_own_branch(self.currentItem(), target)
+        if ok:
+            self._set_drop_target(target)
+        else:
+            self._clear_drop_target()
+
+    @staticmethod
+    def _is_own_branch(dragged, item) -> bool:
+        """True if ``item`` is the dragged row or inside its subtree."""
+        while item is not None:
+            if item is dragged:
+                return True
+            item = item.parent()
+        return False
+
     def _set_drop_target(self, item):
-        """Highlight the playlist row a drop would land on (cleared on the way
+        """Highlight the row a drop would land on (cleared on the way
         out). Foreground is set too so the name stays legible on the highlight."""
         if item is self._drop_target:
             return
@@ -463,18 +492,16 @@ class PlaylistTree(QTreeWidget):
             self._autoscroll_timer.stop()
             return
         # The cursor may be held still while rows scroll under it, so keep the
-        # drop-target highlight on the row now beneath it (track drags only;
-        # internal playlist moves never set a drop target).
-        if self._autoscroll_pos is not None and self._drop_target is not None:
-            target = self.itemAt(self._autoscroll_pos)
-            if target and target.data(0, KIND_ROLE) == "playlist":
-                self._set_drop_target(target)
-            else:
-                self._clear_drop_target()
+        # drop-target highlight on the row now beneath it.
+        if (self._autoscroll_pos is not None
+                and self._drag_target_kind is not None):
+            self._highlight_drop_target(self._autoscroll_pos,
+                                        self._drag_target_kind)
 
     def _stop_autoscroll(self):
         self._autoscroll_dir = 0
         self._autoscroll_pos = None
+        self._drag_target_kind = None
         self._autoscroll_timer.stop()
 
     def dropEvent(self, event):