tree: implement tile dragging

src/tree/placeholder.rs

@@ -10,7 +10,8 @@ use {
         state::State,
         text::TextTexture,
         tree::{
-            Direction, FindTreeResult, FindTreeUsecase, FoundNode, Node, NodeId, NodeVisitor,
+            default_tile_drag_destination, ContainerSplit, Direction, FindTreeResult,
+            FindTreeUsecase, FoundNode, Node, NodeId, NodeVisitor, TileDragDestination,
             ToplevelData, ToplevelNode, ToplevelNodeBase,
         },
         utils::{
@@ -62,6 +63,17 @@ impl PlaceholderNode {
         }
     }
 
+    pub fn new_empty(state: &Rc<State>) -> Self {
+        Self {
+            id: state.node_ids.next(),
+            toplevel: ToplevelData::new(state, String::new(), None),
+            destroyed: Default::default(),
+            update_textures_scheduled: Default::default(),
+            state: state.clone(),
+            textures: Default::default(),
+        }
+    }
+
     pub fn is_destroyed(&self) -> bool {
         self.destroyed.get()
     }
@@ -222,4 +234,15 @@ impl ToplevelNodeBase for PlaceholderNode {
     fn tl_admits_children(&self) -> bool {
         false
     }
+
+    fn tl_tile_drag_destination(
+        self: Rc<Self>,
+        source: NodeId,
+        split: Option<ContainerSplit>,
+        abs_bounds: Rect,
+        x: i32,
+        y: i32,
+    ) -> Option<TileDragDestination> {
+        default_tile_drag_destination(self, source, split, abs_bounds, x, y)
+    }
 }