Update

Author: 0x5bfa

components/Sizers/src/ContentSizer/ContentSizer.cs

@@ -9,4 +9,11 @@ namespace CommunityToolkit.WinUI.Controls;
 /// </summary>
 public partial class ContentSizer : SizerBase
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ContentSizer"/> class.
+    /// </summary>
+    public ContentSizer()
+    {
+        DefaultStyleKey = typeof(ContentSizer);
+    }
 }

components/Sizers/src/GridSplitter/GridSplitter.cs

@@ -142,4 +142,12 @@ private RowDefinition? SiblingRow
             return null;
         }
     }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="GridSplitter"/> class.
+    /// </summary>
+    public GridSplitter()
+    {
+        DefaultStyleKey = typeof(GridSplitter);
+    }
 }

components/Sizers/src/PropertySizer/PropertySizer.cs

@@ -9,4 +9,11 @@ namespace CommunityToolkit.WinUI.Controls;
 /// </summary>
 public partial class PropertySizer : SizerBase
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="PropertySizer"/> class.
+    /// </summary>
+    public PropertySizer()
+    {
+        DefaultStyleKey = typeof(PropertySizer);
+    }
 }

components/Sizers/src/SizerBase.xaml

@@ -47,6 +47,8 @@
     <x:Double x:Key="SizerBaseThumbRadius">2</x:Double>
     <Thickness x:Key="SizerBasePadding">4</Thickness>
 
+    <Style BasedOn="{StaticResource DefaultSizerBaseStyle}"
+           TargetType="controls:SizerBase" />
     <Style BasedOn="{StaticResource DefaultContentSizerStyle}"
            TargetType="controls:ContentSizer" />
     <Style BasedOn="{StaticResource DefaultGridSplitterStyle}"