Experimenting with placing _load_directory in a worker

This isn't the final form, not even close, this is more to help test out the
idea and how well it will work. Note the very deliberate sleep in the code
that's there to emulate loading from a slow blocking source.

This will be removed and tidied up before a final PR, of course. The main
aim here is to emulate a worst-case scenario so that the use of a worker can
be tried out with some confidence.

See Textualize#2456.

src/textual/widgets/_directory_tree.py

@@ -7,6 +7,7 @@
 from rich.style import Style
 from rich.text import Text, TextType
 
+from .. import work
 from ..events import Mount
 from ..message import Message
 from ..reactive import var
@@ -238,10 +239,18 @@ def _directory_content(self, directory: Path) -> Iterator[Path]:
         Returns:
             An iterator of `Path` objects.
         """
-        # TODO: Place this in a loop with a sleep to slow things down for
-        # testing.
-        return directory.iterdir()
-
+        # TODO: Not like this. Oh so very not like this. This is here to
+        # slow things down on purpose, to emulate loading directory
+        # information from a slow source.
+        #
+        # REMOVE BEFORE FLIGHT!
+        import time
+
+        for entry in directory.iterdir():
+            yield entry
+            time.sleep(0.05)
+
+    @work
     def _load_directory(self, node: TreeNode[DirEntry]) -> None:
         """Load the directory contents for a given node.
 
@@ -255,12 +264,13 @@ def _load_directory(self, node: TreeNode[DirEntry]) -> None:
             key=lambda path: (not path.is_dir(), path.name.lower()),
         )
         for path in directory:
-            node.add(
+            self.app.call_from_thread(
+                node.add,
                 path.name,
                 data=DirEntry(path),
                 allow_expand=path.is_dir(),
             )
-        node.expand()
+        self.app.call_from_thread(node.expand)
 
     def _on_mount(self, _: Mount) -> None:
         self._load_directory(self.root)