dnd: Style no-drop icons differently

Add a .no-drop style class to drag icons if there is no selected
action, and style the icon to be translucent in that case, to
indicate 'no drop'.

Related: #6337

gtk/gtkdragicon.c

@@ -399,6 +399,17 @@ gtk_drag_icon_init (GtkDragIcon *self)
   gtk_widget_set_can_target (GTK_WIDGET (self), FALSE);
 }
 
+static void
+selected_action_changed (GdkDrag    *drag,
+                         GParamSpec *pspec,
+                         GtkWidget  *self)
+{
+  if (gdk_drag_get_selected_action (drag) == 0)
+    gtk_widget_add_css_class (self, "no-drop");
+  else
+    gtk_widget_remove_css_class (self, "no-drop");
+}
+
 /**
  * gtk_drag_icon_get_for_drag: (constructor)
  * @drag: a `GdkDrag`
@@ -428,6 +439,9 @@ gtk_drag_icon_get_for_drag (GdkDrag *drag)
 
       GTK_DRAG_ICON (self)->surface = g_object_ref (gdk_drag_get_drag_surface (drag));
 
+      g_signal_connect_object (drag, "notify::selected-action", G_CALLBACK (selected_action_changed), self, 0);
+      selected_action_changed (drag, NULL, self);
+
       g_object_set_qdata_full (G_OBJECT (drag), drag_icon_quark, g_object_ref_sink (self), g_object_unref);
 
       if (GTK_DRAG_ICON (self)->child != NULL)

gtk/theme/Default/_common.scss

@@ -31,6 +31,10 @@ dnd {
   color: $fg-color;
 }
 
+dnd.no-drop {
+  filter: opacity(0.5);
+}
+
 .normal-icons {
   -gtk-icon-size: 16px;
 }