Implement secondary button span but disable it with commented reasons

src/Avalonia.Controls/SplitButton/SplitButton.cs

@@ -186,8 +186,24 @@ namespace Avalonia.Controls
         protected void UpdatePseudoClasses()
         {
             // Place the secondary button
-            // These are mutually exclusive PseudoClasses handled separately from SetExclusivePseudoClass().
+            //
-            // They must be applied in addition to the others.
+            // In WinUI, the span of the secondary button is changed to full-width for touch-based
+            // devices in certain conditions. The full reasoning for this is unknown. Some theories
+            // include:
+            //
+            //   My guess is that the design team at MS decided that it's a better experience
+            //   for touch users to make them select from the drop down rather than the shortcut
+            //   top level button, so touch basically just turns this into a drop down button.
+            //   Whether that's ideal or not is going is a subjective opinion.
+            //
+            // For Avalonia, it may not always make sense to disable the primary button like that
+            // on touch-first platforms. Users and developers would normally expect a control to
+            // function the same on all platforms. Therefore, this functionality is disabled here
+            // but could be re-enabled in the future if more reasons become known.
+            //
+            // Finally, these are mutually exclusive PseudoClasses handled separately from
+            // SetExclusivePseudoClass(). They must be applied in addition to the others.
+            /*
             if (_lastPointerType == PointerType.Touch || _isKeyDown)
             {
                 PseudoClasses.Set(pcSecondaryButtonSpan, true);
@@ -198,9 +214,10 @@ namespace Avalonia.Controls
                 PseudoClasses.Set(pcSecondaryButtonSpan, false);
                 PseudoClasses.Set(pcSecondaryButtonRight, true);
             }
+            */
 
             // Change the visual state
-            if (!IsEnabled)
+            if (!IsEffectivelyEnabled)
             {
                 SetExclusivePseudoClass(pcDisabled);
             }

src/Avalonia.Themes.Fluent/Controls/SplitButton.xaml

@@ -10,7 +10,7 @@
 
   <Styles.Resources>
     <x:Double x:Key="SplitButtonPrimaryButtonSize">32</x:Double>
-    <x:Double x:Key="SplitButtonSecondaryButtonSize">32</x:Double>
+    <GridLength x:Key="SplitButtonSecondaryButtonSize">32</GridLength>
   </Styles.Resources>
 
   <Style Selector="SplitButton">
@@ -43,9 +43,9 @@
               
               <Grid.ColumnDefinitions>
                 <ColumnDefinition Width="*"
-                                  MinWidth="{DynamicResource SplitButtonPrimaryButtonSize}" />
+                                  MinWidth="{StaticResource SplitButtonPrimaryButtonSize}" />
                 <ColumnDefinition Width="1" />
-                <ColumnDefinition Width="{DynamicResource SplitButtonSecondaryButtonSize}" />
+                <ColumnDefinition Width="{StaticResource SplitButtonSecondaryButtonSize}" />
               </Grid.ColumnDefinitions>
 
               <!-- Changed from Grid to Rectangle since Avalonia does not support Background on Grid directly -->
@@ -89,11 +89,12 @@
                       Background="{TemplateBinding Background}"
                       BorderThickness="{TemplateBinding BorderThickness}"
                       BorderBrush="Transparent"
-                      HorizontalContentAlignment="Center"
+                      HorizontalContentAlignment="Right"
                       VerticalContentAlignment="Center"
                       HorizontalAlignment="Stretch"
                       VerticalAlignment="Stretch"
-                      KeyboardNavigation.IsTabStop="False">
+                      KeyboardNavigation.IsTabStop="False"
+                      Padding="0,0,10,0">
                 <Button.Content>
                   <!-- TextBlock converted to a Path and placed within Viewbox -->
                   <Viewbox UseLayoutRounding="False"
@@ -384,10 +385,16 @@
   </Style>
 
   <!-- SecondaryButtonPlacement States -->
-  <!-- Note: ':secondary-button-right' is the default -->
+  <!-- Note: Setting Grid attached properties doesn't work here for some unknown reason.
-  <!--<Style Selector="SplitButton:secondary-button-span /template/ Button#PART_SecondaryButton">
+       They simply are never changed. In addition, touch-device changes of secondary button
+       size are currently disabled in code-behind. -->
+  <Style Selector="SplitButton:secondary-button-right /template/ Button#PART_SecondaryButton">
+    <Setter Property="(Grid.Column)" Value="2" />
+    <Setter Property="(Grid.ColumnSpan)" Value="1" />
+  </Style>
+  <Style Selector="SplitButton:secondary-button-span /template/ Button#PART_SecondaryButton">
     <Setter Property="(Grid.Column)" Value="0" />
     <Setter Property="(Grid.ColumnSpan)" Value="3" />
-  </Style>-->
+  </Style>
 
 </Styles>