diff --git a/android/src/toga_android/widgets/box.py b/android/src/toga_android/widgets/box.py
index ef2c0eae9b..d4f0ad112b 100644
--- a/android/src/toga_android/widgets/box.py
+++ b/android/src/toga_android/widgets/box.py
@@ -10,9 +10,6 @@ def create(self):
         self.native = RelativeLayout(MainActivity.singletonThis)
 
     def set_child_bounds(self, widget, x, y, width, height):
-        # Avoid setting child boundaries if `create()` has not been called.
-        if not widget.native:
-            return
         # We assume `widget.native` has already been added to this `RelativeLayout`.
         #
         # We use `topMargin` and `leftMargin` to perform absolute layout. Not very
diff --git a/android/tests_backend/widgets/base.py b/android/tests_backend/widgets/base.py
index f0bfab91fa..ee6be9499d 100644
--- a/android/tests_backend/widgets/base.py
+++ b/android/tests_backend/widgets/base.py
@@ -5,6 +5,8 @@
 from android.view import ViewTreeObserver
 from toga.fonts import SYSTEM
 
+from .properties import toga_color
+
 
 class LayoutListener(dynamic_proxy(ViewTreeObserver.OnGlobalLayoutListener)):
     def __init__(self):
@@ -80,5 +82,9 @@ def height(self):
         # Return the value in DP
         return self.native.getHeight() / self.scale_factor
 
+    @property
+    def background_color(self):
+        return toga_color(self.native.getBackground().getColor())
+
     def press(self):
         self.native.performClick()
diff --git a/android/tests_backend/widgets/box.py b/android/tests_backend/widgets/box.py
new file mode 100644
index 0000000000..9db2709ccc
--- /dev/null
+++ b/android/tests_backend/widgets/box.py
@@ -0,0 +1,7 @@
+from java import jclass
+
+from .base import SimpleProbe
+
+
+class BoxProbe(SimpleProbe):
+    native_class = jclass("android.widget.RelativeLayout")
diff --git a/android/tests_backend/widgets/label.py b/android/tests_backend/widgets/label.py
index 1598da5c77..c8708cef07 100644
--- a/android/tests_backend/widgets/label.py
+++ b/android/tests_backend/widgets/label.py
@@ -11,10 +11,6 @@ class LabelProbe(SimpleProbe):
     def color(self):
         return toga_color(self.native.getCurrentTextColor())
 
-    @property
-    def background_color(self):
-        return toga_color(self.native.getBackground().getColor())
-
     @property
     def text(self):
         return str(self.native.getText())
diff --git a/changes/1820.feature.rst b/changes/1820.feature.rst
new file mode 100644
index 0000000000..8bc73e8e69
--- /dev/null
+++ b/changes/1820.feature.rst
@@ -0,0 +1 @@
+The Box widget now has 100% test coverage, and complete API documentation.
diff --git a/cocoa/src/toga_cocoa/widgets/box.py b/cocoa/src/toga_cocoa/widgets/box.py
index bc5454dd65..df5ebec834 100644
--- a/cocoa/src/toga_cocoa/widgets/box.py
+++ b/cocoa/src/toga_cocoa/widgets/box.py
@@ -11,11 +11,6 @@ def isFlipped(self) -> bool:
         # Default Cocoa coordinate frame is around the wrong way.
         return True
 
-    @objc_method
-    def display(self) -> None:
-        self.layer.needsDisplay = True
-        self.layer.displayIfNeeded()
-
 
 class Box(Widget):
     def create(self):
diff --git a/cocoa/tests_backend/widgets/base.py b/cocoa/tests_backend/widgets/base.py
index bad58265cf..09dbce14ee 100644
--- a/cocoa/tests_backend/widgets/base.py
+++ b/cocoa/tests_backend/widgets/base.py
@@ -1,7 +1,10 @@
 import asyncio
 
+from toga.colors import TRANSPARENT
 from toga.fonts import CURSIVE, FANTASY, MONOSPACE, SANS_SERIF, SERIF, SYSTEM
 
+from .properties import toga_color
+
 
 class SimpleProbe:
     def __init__(self, widget):
@@ -55,5 +58,15 @@ def width(self):
     def height(self):
         return self.native.frame.size.height
 
+    @property
+    def background_color(self):
+        if self.native.drawsBackground:
+            if self.native.backgroundColor:
+                return toga_color(self.native.backgroundColor)
+            else:
+                return None
+        else:
+            return TRANSPARENT
+
     def press(self):
         self.native.performClick(None)
diff --git a/cocoa/tests_backend/widgets/box.py b/cocoa/tests_backend/widgets/box.py
new file mode 100644
index 0000000000..0bc31e433e
--- /dev/null
+++ b/cocoa/tests_backend/widgets/box.py
@@ -0,0 +1,7 @@
+from toga_cocoa.libs import NSView
+
+from .base import SimpleProbe
+
+
+class BoxProbe(SimpleProbe):
+    native_class = NSView
diff --git a/cocoa/tests_backend/widgets/label.py b/cocoa/tests_backend/widgets/label.py
index b26fdfac6b..d4faa7e066 100644
--- a/cocoa/tests_backend/widgets/label.py
+++ b/cocoa/tests_backend/widgets/label.py
@@ -1,4 +1,3 @@
-from toga.colors import TRANSPARENT
 from toga_cocoa.libs import NSTextField
 
 from .base import SimpleProbe
@@ -16,16 +15,6 @@ def text(self):
     def color(self):
         return toga_color(self.native.textColor)
 
-    @property
-    def background_color(self):
-        if self.native.drawsBackground:
-            if self.native.backgroundColor:
-                return toga_color(self.native.backgroundColor)
-            else:
-                return None
-        else:
-            return TRANSPARENT
-
     @property
     def font(self):
         return toga_font(self.native.font)
diff --git a/core/src/toga/widgets/box.py b/core/src/toga/widgets/box.py
index 4001127020..97ba28a080 100644
--- a/core/src/toga/widgets/box.py
+++ b/core/src/toga/widgets/box.py
@@ -1,38 +1,24 @@
-import warnings
-
 from .base import Widget
 
 
 class Box(Widget):
-    """This is a Widget that contains other widgets, but has no rendering or
-    interaction of its own.
-
-    Args:
-        id (str): An identifier for this widget.
-        style (:class:~colosseum.CSSNode`): An optional style object. If no
-            style is provided then a new one will be created for the widget.
-        children (``list`` of :class:`~toga.Widget`):  An optional list of child
-            Widgets that will be in this box.
-    """
-
     def __init__(
         self,
         id=None,
         style=None,
         children=None,
-        factory=None,  # DEPRECATED!
     ):
-        super().__init__(id=id, style=style)
+        """Create a new Box container widget.
+
+        Inherits from :class:`~toga.widgets.base.Widget`.
 
-        ######################################################################
-        # 2022-09: Backwards compatibility
-        ######################################################################
-        # factory no longer used
-        if factory:
-            warnings.warn("The factory argument is no longer used.", DeprecationWarning)
-        ######################################################################
-        # End backwards compatibility.
-        ######################################################################
+        :param text: The text to display on the button.
+        :param id: The ID for the widget.
+        :param style: A style object. If no style is provided, a default style
+            will be applied to the widget.
+        :param children: An optional list of children for to add to the Box.
+        """
+        super().__init__(id=id, style=style)
 
         self._children = []
         if children:
diff --git a/core/tests/test_deprecated_factory.py b/core/tests/test_deprecated_factory.py
index 2a434b873f..4d1713ad96 100644
--- a/core/tests/test_deprecated_factory.py
+++ b/core/tests/test_deprecated_factory.py
@@ -76,12 +76,6 @@ def test_window(self):
         self.assertEqual(widget._impl.interface, widget)
         self.assertNotEqual(widget.factory, self.factory)
 
-    def test_box_created(self):
-        with self.assertWarns(DeprecationWarning):
-            widget = toga.Box(children=[toga.Widget()], factory=self.factory)
-        self.assertEqual(widget._impl.interface, widget)
-        self.assertNotEqual(widget.factory, self.factory)
-
     def test_canvas_created(self):
         with self.assertWarns(DeprecationWarning):
             widget = toga.Canvas(factory=self.factory)
diff --git a/core/tests/widgets/test_box.py b/core/tests/widgets/test_box.py
index c0f41a2054..bfd5e2a57d 100644
--- a/core/tests/widgets/test_box.py
+++ b/core/tests/widgets/test_box.py
@@ -1,16 +1,33 @@
 import toga
-from toga_dummy.utils import TestCase
+from toga_dummy.utils import (
+    assert_action_not_performed,
+    assert_action_performed,
+)
 
 
-class BoxTests(TestCase):
-    def setUp(self):
-        super().setUp()
-        self.children = [toga.Widget()]
-        self.box = toga.Box(children=self.children)
+def test_create_box():
+    "A Box can be created."
+    box = toga.Box()
+    # Round trip the impl/interface
+    assert box._impl.interface == box
 
-    def test_widget_created(self):
-        self.assertEqual(self.box._impl.interface, self.box)
-        self.assertActionPerformed(self.box, "create Box")
+    assert_action_performed(box, "create Box")
+    assert_action_not_performed(box, "add child")
 
-    def test_children_added(self):
-        self.assertEqual(self.box._children, self.children)
+
+def test_create_box_with_children():
+    "A Box can be created with children."
+    child1 = toga.Box()
+    child2 = toga.Box()
+    box = toga.Box(children=[child1, child2])
+
+    # Round trip the impl/interface
+    assert box._impl.interface == box
+
+    assert_action_performed(box, "create Box")
+    # The impl-level add-child will not be called,
+    # because the box hasn't been assigned to a window
+    assert_action_not_performed(box, "add child")
+
+    # But the box will have children.
+    assert box.children == [child1, child2]
diff --git a/docs/reference/api/containers/box.rst b/docs/reference/api/containers/box.rst
index 2b7c8ea2a1..3330d0afa8 100644
--- a/docs/reference/api/containers/box.rst
+++ b/docs/reference/api/containers/box.rst
@@ -1,6 +1,8 @@
 Box
 ===
 
+A generic container for other widgets. Used to construct layouts.
+
 .. rst-class:: widget-support
 .. csv-filter:: Availability (:ref:`Key <api-status-key>`)
    :header-rows: 1
@@ -8,57 +10,40 @@ Box
    :included_cols: 4,5,6,7,8,9
    :exclude: {0: '(?!(Box|Component))'}
 
-The box is a generic container for widgets, allowing you to construct layouts.
-
 Usage
 -----
 
-A box can be instantiated with no children and the children added later:
-
-.. code-block:: Python
-
-    import toga
-
-    box = toga.Box('box1')
-
-    button = toga.Button('Hello world', on_press=button_handler)
-    box.add(button)
-
-To create boxes within boxes, use the children argument:
+An empty Box can be constructed without any children, with children added to the
+box after construction:
 
 .. code-block:: Python
 
     import toga
 
-    box_a = toga.Box('box_a')
-    box_b = toga.Box('box_b')
+    box = toga.Box()
 
-    box = toga.Box('box', children=[box_a, box_b])
+    label1 = toga.Label('Hello')
+    label2 = toga.Label('World')
 
-Box Styling
------------
+    box.add(label1)
+    box.add(label2)
 
-Styling of boxes can be done during instantiation of the Box:
+Alternatively, children can be specified at the time the box is constructed:
 
 .. code-block:: Python
 
     import toga
-    from toga.style import Pack
-    from toga.style.pack import COLUMN
-
-    box = toga.Box(id='box', style=Pack(direction=COLUMN, padding_top=10))
-
-Styles can be also be updated on an existing instance:
 
-.. code-block:: Python
-
-    import toga
-    from toga.style import Pack
-    from toga.style.pack import COLUMN
+    label1 = toga.Label('Hello')
+    label2 = toga.Label('World')
 
-    box = toga.Box(id='box', style=Pack(direction=COLUMN))
+    box = toga.Box(children=[label1, label2])
 
-    box.style.update(padding_top=10)
+In most apps, a layout is constructed by building a tree of boxes inside boxes,
+with concrete widgets (such as :class:`~toga.widgets.label.Label` or
+:class:`~toga.widgets.button.Button`) forming the leaf nodes of the tree. Style
+directives can be applied to enforce padding around the outside of the box,
+direction of child stacking inside the box, and background color of the box.
 
 Reference
 ---------
diff --git a/docs/reference/data/widgets_by_platform.csv b/docs/reference/data/widgets_by_platform.csv
index c56e983beb..aa63a35b2e 100644
--- a/docs/reference/data/widgets_by_platform.csv
+++ b/docs/reference/data/widgets_by_platform.csv
@@ -23,7 +23,7 @@ TimePicker,General Widget,:class:`~toga.widgets.timepicker.TimePicker`,An input
 Tree,General Widget,:class:`~toga.widgets.tree.Tree`,Tree of data,|b|,|b|,|b|,,,
 WebView,General Widget,:class:`~toga.widgets.webview.WebView`,A panel for displaying HTML,|b|,|b|,|b|,|b|,|b|,
 Widget,General Widget,:class:`~toga.widgets.base.Widget`,The base widget,|b|,|b|,|b|,|b|,|b|,|b|
-Box,Layout Widget,:class:`~toga.widgets.box.Box`,Container for components,|b|,|b|,|b|,|b|,|b|,|b|
+Box,Layout Widget,:class:`~toga.widgets.box.Box`,Container for components,|y|,|y|,|y|,|y|,|y|,|b|
 ScrollContainer,Layout Widget,:class:`~toga.widgets.scrollcontainer.ScrollContainer`,Scrollable Container,|b|,|b|,|b|,|b|,|b|,
 SplitContainer,Layout Widget,:class:`~toga.widgets.splitcontainer.SplitContainer`,Split Container,|b|,|b|,|b|,,,
 OptionContainer,Layout Widget,:class:`~toga.widgets.optioncontainer.OptionContainer`,Option Container,|b|,|b|,|b|,,,
diff --git a/iOS/src/toga_iOS/widgets/box.py b/iOS/src/toga_iOS/widgets/box.py
index ad83b30e61..fa57d6542c 100644
--- a/iOS/src/toga_iOS/widgets/box.py
+++ b/iOS/src/toga_iOS/widgets/box.py
@@ -1,28 +1,12 @@
-from rubicon.objc import objc_method, objc_property
 from travertino.size import at_least
 
 from toga_iOS.libs import UIView
 from toga_iOS.widgets.base import Widget
 
 
-class TogaView(UIView):
-    interface = objc_property(object, weak=True)
-    impl = objc_property(object, weak=True)
-
-    @objc_method
-    def isFlipped(self) -> bool:
-        # Default Cocoa coordinate frame is around the wrong way.
-        return True
-
-    @objc_method
-    def display(self) -> None:
-        self.layer.setNeedsDisplay_(True)
-        self.layer.displayIfNeeded()
-
-
 class Box(Widget):
     def create(self):
-        self.native = TogaView.alloc().init()
+        self.native = UIView.alloc().init()
         self.native.interface = self.interface
         self.native.impl = self
 
diff --git a/iOS/tests_backend/widgets/base.py b/iOS/tests_backend/widgets/base.py
index db2ce267a1..3b40650e09 100644
--- a/iOS/tests_backend/widgets/base.py
+++ b/iOS/tests_backend/widgets/base.py
@@ -1,7 +1,10 @@
 import asyncio
 
+from toga.colors import TRANSPARENT
 from toga.fonts import CURSIVE, FANTASY, MONOSPACE, SANS_SERIF, SERIF, SYSTEM
-from toga_iOS.libs import NSRunLoop
+from toga_iOS.libs import NSRunLoop, UIColor
+
+from .properties import toga_color
 
 # From UIControl.h
 UIControlEventTouchDown = 1 << 0
@@ -87,5 +90,12 @@ def width(self):
     def height(self):
         return self.native.frame.size.height
 
+    @property
+    def background_color(self):
+        if self.native.backgroundColor == UIColor.clearColor:
+            return TRANSPARENT
+        else:
+            return toga_color(self.native.backgroundColor)
+
     def press(self):
         self.native.sendActionsForControlEvents(UIControlEventTouchDown)
diff --git a/iOS/tests_backend/widgets/box.py b/iOS/tests_backend/widgets/box.py
new file mode 100644
index 0000000000..32e111f4e3
--- /dev/null
+++ b/iOS/tests_backend/widgets/box.py
@@ -0,0 +1,7 @@
+from toga_iOS.libs import UIView
+
+from .base import SimpleProbe
+
+
+class BoxProbe(SimpleProbe):
+    native_class = UIView
diff --git a/iOS/tests_backend/widgets/label.py b/iOS/tests_backend/widgets/label.py
index e2d870bbdc..b0c97a24a9 100644
--- a/iOS/tests_backend/widgets/label.py
+++ b/iOS/tests_backend/widgets/label.py
@@ -1,5 +1,4 @@
-from toga.colors import TRANSPARENT
-from toga_iOS.libs import UIColor, UILabel
+from toga_iOS.libs import UILabel
 
 from .base import SimpleProbe
 from .properties import toga_alignment, toga_color, toga_font
@@ -19,13 +18,6 @@ def text(self):
     def color(self):
         return toga_color(self.native.textColor)
 
-    @property
-    def background_color(self):
-        if self.native.backgroundColor == UIColor.clearColor:
-            return TRANSPARENT
-        else:
-            return toga_color(self.native.backgroundColor)
-
     @property
     def font(self):
         return toga_font(self.native.font)
diff --git a/testbed/tests/widgets/properties.py b/testbed/tests/widgets/properties.py
index ca0d967170..17671d9d26 100644
--- a/testbed/tests/widgets/properties.py
+++ b/testbed/tests/widgets/properties.py
@@ -1,3 +1,5 @@
+from pytest import approx
+
 from toga.colors import RED, TRANSPARENT, color as named_color
 from toga.fonts import BOLD, FANTASY, ITALIC, NORMAL, SERIF, SYSTEM
 from toga.style.pack import COLUMN
@@ -147,6 +149,50 @@ async def test_background_color_transparent(widget, probe):
     assert_color(probe.background_color, TRANSPARENT)
 
 
+async def test_flex_widget_size(widget, probe):
+    "The widget can expand in either axis."
+    # Container is initially a non-flex row box. Paint it red so we can see it.
+    widget.style.background_color = RED
+    await probe.redraw()
+
+    # Check the initial widget size
+    # Match isn't exact because of pixel scaling on some platforms
+    assert probe.width == approx(100, rel=0.01)
+    assert probe.height == approx(100, rel=0.01)
+
+    # Drop the fixed height, and make the widget flexible
+    widget.style.flex = 1
+    del widget.style.height
+
+    # Widget should now be 100 pixels wide, but as tall as the container.
+    await probe.redraw()
+    assert probe.width == approx(100, rel=0.01)
+    assert probe.height > 300
+
+    # Make the parent a COLUMN box
+    del widget.style.width
+    widget.parent.style.direction = COLUMN
+
+    # Widget should now be the size of the container
+    await probe.redraw()
+    assert probe.width > 300
+    assert probe.height > 300
+
+    # Revert to fixed height
+    widget.style.height = 150
+
+    await probe.redraw()
+    assert probe.width > 300
+    assert probe.height == approx(150, rel=0.01)
+
+    # Revert to fixed width
+    widget.style.width = 150
+
+    await probe.redraw()
+    assert probe.width == approx(150, rel=0.01)
+    assert probe.height == approx(150, rel=0.01)
+
+
 async def test_flex_horizontal_widget_size(widget, probe):
     "Check that a widget that is flexible in the horizontal axis resizes as expected"
     # Container is initially a non-flex row box.
diff --git a/testbed/tests/widgets/test_box.py b/testbed/tests/widgets/test_box.py
new file mode 100644
index 0000000000..93efac2b6e
--- /dev/null
+++ b/testbed/tests/widgets/test_box.py
@@ -0,0 +1,15 @@
+import pytest
+
+import toga
+from toga.style import Pack
+
+from .properties import (  # noqa: F401
+    test_background_color,
+    test_background_color_reset,
+    test_flex_widget_size,
+)
+
+
+@pytest.fixture
+async def widget():
+    return toga.Box(style=Pack(width=100, height=100))