✨ Add testing code for tree lines

Textualize/textual#2397

tree_lines_tester.py

@@ -0,0 +1,38 @@
+"""https://github.com/Textualize/textual/issues/2397"""
+
+from textual.app          import App, ComposeResult
+from textual.containers   import Horizontal
+from textual.widgets      import Header, Footer, Tree
+from textual.widgets.tree import TreeNode
+
+class TreeLinesTestApp( App[ None ] ):
+
+    CSS = """
+    Tree {
+        border: round red;
+    }
+
+    Tree:focus {
+        border: double green;
+    }
+    """
+
+    def compose( self ) -> ComposeResult:
+        yield Header()
+        with Horizontal():
+            yield Tree("Root", id="root")
+            yield Tree("Root", id="no-root")
+        yield Footer()
+
+    def populate( self, node: TreeNode, limit: int=3 ) -> Tree:
+        for n in range( limit ):
+            self.populate( node.add( str( n ) ), limit-1 )
+        return node.tree
+
+    def on_mount( self ) -> None:
+        self.query_one( "#no-root", Tree ).show_root = False
+        self.populate( self.query_one( "#root", Tree ).root ).root.expand_all()
+        self.populate( self.query_one( "#no-root", Tree ).root ).root.expand_all()
+
+if __name__ == "__main__":
+    TreeLinesTestApp().run()