Ensure that TreeNode messages are posted when via-API changes are made

Until now the Tree.NodeExpanded and Tree.NodeCollapsed messages were only
sent out when changes were made to the tree by user interaction. This meant
that if any changes were made with the TreeNode expand, expand_all,
collapse, collapse_all, toggle or toggle_all API calls no messages would be
sent.

This PR corrects this.

The work here is, in part, required for #2456 (DirectoryTree lazy-loads
directory information on node expansion so if someone is expanding nodes
under code control the DirectoryTree never gets to know that it should load
a directory's content) and will build on #1644, essentially adding a missing
aspect to the latter PR.

tests/tree/test_tree_messages.py

@@ -78,6 +78,20 @@ async def test_tree_node_expanded_message() -> None:
         assert pilot.app.messages == [("NodeExpanded", "test-tree")]
 
 
+async def tree_node_expanded_by_code_message() -> None:
+    """Expanding a node via the API should result in an expanded message being posted."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].expand()
+        assert pilot.app.messages == [("NodeExpanded", "test-tree")]
+
+
+async def tree_node_all_expanded_by_code_message() -> None:
+    """Expanding all nodes via the API should result in expanded messages being posted."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].expand_all()
+        assert pilot.app.messages == [("NodeExpanded", "test-tree")]
+
+
 async def test_tree_node_collapsed_message() -> None:
     """Collapsing a node should result in a collapsed message being emitted."""
     async with TreeApp().run_test() as pilot:
@@ -89,6 +103,46 @@ async def test_tree_node_collapsed_message() -> None:
         ]
 
 
+async def tree_node_collapsed_by_code_message() -> None:
+    """Collapsing a node via the API should result in a collapsed message being posted."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].expand().collapse()
+        assert pilot.app.messages == [
+            ("NodeExpanded", "test-tree"),
+            ("NodeCollapsed", "test-tree"),
+        ]
+
+
+async def tree_node_all_collapsed_by_code_message() -> None:
+    """Collapsing all nodes via the API should result in collapsed messages being posted."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].expand_all().collapse_all()
+        assert pilot.app.messages == [
+            ("NodeExpanded", "test-tree"),
+            ("NodeCollapsed", "test-tree"),
+        ]
+
+
+async def tree_node_toggled_by_code_message() -> None:
+    """Toggling a node twice via the API should result in expanded and collapsed messages."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].toggle().toggle()
+        assert pilot.app.messages == [
+            ("NodeExpanded", "test-tree"),
+            ("NodeCollapsed", "test-tree"),
+        ]
+
+
+async def tree_node_all_toggled_by_code_message() -> None:
+    """Toggling all nodes twice via the API should result in expanded and collapsed messages."""
+    async with TreeApp().run_test() as pilot:
+        pilot.app.query_one(Tree).root.children[0].toggle_all().toggle_all()
+        assert pilot.app.messages == [
+            ("NodeExpanded", "test-tree"),
+            ("NodeCollapsed", "test-tree"),
+        ]
+
+
 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: