optimization

src/textual/_arrange.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-
+from collections import defaultdict
 from fractions import Fraction
 from operator import attrgetter
 from typing import Sequence, TYPE_CHECKING
@@ -33,11 +33,10 @@ def arrange(
 
     arrange_widgets: set[Widget] = set()
 
-    dock_layers: dict[str, list[Widget]] = {}
-    dock_layers_setdefault = dock_layers.setdefault
+    dock_layers: defaultdict[str, list[Widget]] = defaultdict(list)
     for child in children:
         if child.display:
-            dock_layers_setdefault(child.styles.layer or "default", []).append(child)
+            dock_layers[child.styles.layer or "default"].append(child)
 
     width, height = size