Add a message for when a node is highlighted

This is sort of different from selected. Selected is when someone mashes the
enter button or clicks on a node. Highlighted is when the cursor moves into
a new node.

See Textualize#1400.

src/textual/widgets/_tree.py

@@ -365,6 +365,13 @@ class NodeCollapsed(NodeMessage[EventTreeDataType]):
             TreeNode[EventTreeDataType]: The node that was collapsed.
         """
 
+    class NodeHighlighted(NodeMessage[EventTreeDataType]):
+        """Event sent when a node is highlighted.
+
+        Attributes:
+            TreeNode[EventTreeDataType]: The node that was collapsed.
+        """
+
     def __init__(
         self,
         label: TextType,
@@ -573,6 +580,8 @@ def watch_cursor_line(self, previous_line: int, line: int) -> None:
             self._refresh_node(node)
             node._selected = True
             self._cursor_node = node
+            if previous_node != node:
+                self.post_message_no_wait(self.NodeHighlighted(self, node))
 
     def watch_guide_depth(self, guide_depth: int) -> None:
         self._invalidate()

tests/tree/test_tree_messages.py

@@ -32,6 +32,9 @@ def on_tree_node_expanded(self, event: Tree.NodeExpanded[None]) -> None:
     def on_tree_node_collapsed(self, event: Tree.NodeCollapsed[None]) -> None:
         self.record(event)
 
+    def on_tree_node_highlighted(self, event: Tree.NodeHighlighted[None]) -> None:
+        self.record(event)
+
 
 async def test_tree_node_selected_message() -> None:
     """Selecting a node should result in a selected message being emitted."""
@@ -57,3 +60,10 @@ async def test_tree_node_collapsed_message() -> None:
             "NodeCollapsed",
             "NodeSelected",
         ]
+
+
+async def test_tree_node_highlighted_message() -> None:
+    """Highlighting a node should result in a highlighted message being emitted."""
+    async with TreeApp().run_test() as pilot:
+        await pilot.press("enter", "down")
+        assert pilot.app.messages == ["NodeExpanded", "NodeSelected", "NodeHighlighted"]