diff --git a/src/textual/_arrange.py b/src/textual/_arrange.py
index 8b8b72476..6e133b1d3 100644
--- a/src/textual/_arrange.py
+++ b/src/textual/_arrange.py
@@ -13,6 +13,9 @@ from ._partition import partition
 if TYPE_CHECKING:
     from .widget import Widget
 
+# TODO: This is a bit of a fudge, need to ensure it is impossible for layouts to generate this value
+TOP_Z = 2**31 - 1
+
 
 def arrange(
     widget: Widget, children: Sequence[Widget], size: Size, viewport: Size
@@ -43,10 +46,7 @@ def arrange(
     region = size.region
 
     _WidgetPlacement = WidgetPlacement
-
-    # TODO: This is a bit of a fudge, need to ensure it is impossible for layouts to generate this value
-    top_z = 2**31 - 1
-
+    top_z = TOP_Z
     scroll_spacing = Spacing()
 
     get_dock = attrgetter("styles.dock")
@@ -87,7 +87,7 @@ def arrange(
                 right = max(right, dock_region.width)
             else:
                 # Should not occur, mainly to keep Mypy happy
-                raise AssertionError("invalid value for edge")
+                raise AssertionError("invalid value for edge")  # pragma: no-cover
 
             align_offset = dock_widget.styles.align_size(
                 (widget_width, widget_height), size
diff --git a/src/textual/dom.py b/src/textual/dom.py
index 920cde011..ccccd19e2 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -12,7 +12,7 @@ from rich.tree import Tree
 
 from ._context import NoActiveAppError
 from ._node_list import NodeList
-from .color import Color
+from .color import Color, WHITE, BLACK
 from .css._error_tools import friendly_list
 from .css.constants import VALID_DISPLAY, VALID_VISIBILITY
 from .css.errors import StyleValueError
@@ -379,7 +379,7 @@ class DOMNode(MessagePump):
 
         """
 
-        base_background = background = Color(0, 0, 0, 0)
+        base_background = background = BLACK
 
         for node in reversed(self.ancestors):
             styles = node.styles
@@ -395,8 +395,8 @@ class DOMNode(MessagePump):
         Returns:
             tuple[Color, Color, Color, Color]: Tuple of (base background, base color, background, color)
         """
-        base_background = background = Color(0, 0, 0, 0)
-        base_color = color = Color(255, 255, 255, 0)
+        base_background = background = WHITE
+        base_color = color = BLACK
         for node in reversed(self.ancestors):
             styles = node.styles
             if styles.has_rule("background"):
diff --git a/tests/test_arrange.py b/tests/test_arrange.py
new file mode 100644
index 000000000..16680cc68
--- /dev/null
+++ b/tests/test_arrange.py
@@ -0,0 +1,109 @@
+from textual._arrange import arrange, TOP_Z
+from textual._layout import WidgetPlacement
+from textual.geometry import Region, Size, Spacing
+from textual.widget import Widget
+
+
+def test_arrange_empty():
+    container = Widget(id="container")
+
+    placements, widgets, spacing = arrange(container, [], Size(80, 24), Size(80, 24))
+    assert placements == []
+    assert widgets == set()
+    assert spacing == Spacing(0, 0, 0, 0)
+
+
+def test_arrange_dock_top():
+    container = Widget(id="container")
+    child = Widget(id="child")
+    header = Widget(id="header")
+    header.styles.dock = "top"
+    header.styles.height = "1"
+
+    placements, widgets, spacing = arrange(
+        container, [child, header], Size(80, 24), Size(80, 24)
+    )
+    assert placements == [
+        WidgetPlacement(Region(0, 0, 80, 1), header, order=TOP_Z, fixed=True),
+        WidgetPlacement(Region(0, 1, 80, 23), child, order=0, fixed=False),
+        WidgetPlacement(
+            region=Region(x=0, y=1, width=80, height=23),
+            widget=None,
+            order=0,
+            fixed=False,
+        ),
+    ]
+    assert widgets == {child, header}
+    assert spacing == Spacing(1, 0, 0, 0)
+
+
+def test_arrange_dock_left():
+    container = Widget(id="container")
+    child = Widget(id="child")
+    header = Widget(id="header")
+    header.styles.dock = "left"
+    header.styles.width = "10"
+
+    placements, widgets, spacing = arrange(
+        container, [child, header], Size(80, 24), Size(80, 24)
+    )
+    assert placements == [
+        WidgetPlacement(Region(0, 0, 10, 24), header, order=TOP_Z, fixed=True),
+        WidgetPlacement(Region(10, 0, 70, 24), child, order=0, fixed=False),
+        WidgetPlacement(
+            region=Region(x=10, y=0, width=70, height=24),
+            widget=None,
+            order=0,
+            fixed=False,
+        ),
+    ]
+    assert widgets == {child, header}
+    assert spacing == Spacing(0, 0, 0, 10)
+
+
+def test_arrange_dock_right():
+    container = Widget(id="container")
+    child = Widget(id="child")
+    header = Widget(id="header")
+    header.styles.dock = "right"
+    header.styles.width = "10"
+
+    placements, widgets, spacing = arrange(
+        container, [child, header], Size(80, 24), Size(80, 24)
+    )
+    assert placements == [
+        WidgetPlacement(Region(70, 0, 10, 24), header, order=TOP_Z, fixed=True),
+        WidgetPlacement(Region(0, 0, 70, 24), child, order=0, fixed=False),
+        WidgetPlacement(
+            region=Region(x=0, y=0, width=70, height=24),
+            widget=None,
+            order=0,
+            fixed=False,
+        ),
+    ]
+    assert widgets == {child, header}
+    assert spacing == Spacing(0, 10, 0, 0)
+
+
+def test_arrange_dock_bottom():
+    container = Widget(id="container")
+    child = Widget(id="child")
+    header = Widget(id="header")
+    header.styles.dock = "bottom"
+    header.styles.height = "1"
+
+    placements, widgets, spacing = arrange(
+        container, [child, header], Size(80, 24), Size(80, 24)
+    )
+    assert placements == [
+        WidgetPlacement(Region(0, 23, 80, 1), header, order=TOP_Z, fixed=True),
+        WidgetPlacement(Region(0, 0, 80, 23), child, order=0, fixed=False),
+        WidgetPlacement(
+            region=Region(x=0, y=0, width=80, height=23),
+            widget=None,
+            order=0,
+            fixed=False,
+        ),
+    ]
+    assert widgets == {child, header}
+    assert spacing == Spacing(0, 0, 1, 0)