audgui: Make drag and drop work on Wayland. Closes: audacious-media-p…

…layer#1423

Calling both gtk_drag_get_data() and gtk_drag_finish()
leads to drag and drop not working on Wayland and this error:
"error reading selection buffer: Operation was cancelled"

See also: https://gitlab.gnome.org/GNOME/gimp/-/merge_requests/927

src/libaudgui/list.cc

@@ -488,6 +488,7 @@ static gboolean drag_drop (GtkWidget * widget, GdkDragContext * context, int x,
     g_signal_stop_emission_by_name (widget, "drag-drop");
 
     gboolean success = true;
+    bool finish_drag = true;
     int row = audgui_list_row_at_point_rounded (widget, x, y);
 
     if (model->dragging && MODEL_HAS_CB (model, shift_rows))
@@ -504,11 +505,16 @@ static gboolean drag_drop (GtkWidget * widget, GdkDragContext * context, int x,
         model->receive_row = row;
         gtk_drag_get_data (widget, context, gdk_atom_intern
          (model->cbs->data_type, false), time);
+
+        /* avoid calling both gtk_drag_get_data() and gtk_drag_finish()
+         * to keep drag and drop working on Wayland */
+        finish_drag = false;
     }
     else
         success = false;
 
-    gtk_drag_finish (context, success, false, time);
+    if (finish_drag)
+        gtk_drag_finish (context, success, false, time);
     gtk_tree_view_set_drag_dest_row ((GtkTreeView *) widget, nullptr, (GtkTreeViewDropPosition) 0);
     stop_autoscroll (model, widget);
     return true;