Fix: Allow ColorPicker-Fylout to swap position if needed (#13567)

* Add `PlacementConstraintAdjustmentProperty` to PopupFlyoutBase

This adds much more flexibility to control flyout positioning when there is not enough space to show the Flyout.

* Make Fylout position of the ColorPicker a DynamicResource

This allows the position to be overridden easily. It should no longer be hard coded.

* Address review

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

@@ -1,9 +1,11 @@
-<ResourceDictionary xmlns="https://github.com/avaloniaui"
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:controls="using:Avalonia.Controls"
                     xmlns:primitives="using:Avalonia.Controls.Primitives"
                     x:ClassModifier="internal">
 
+  <PlacementMode x:Key="ColorPickerFlyoutPlacement">Top</PlacementMode>
+  
   <ControlTheme x:Key="{x:Type ColorPicker}"
                 TargetType="ColorPicker">
     <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
@@ -51,7 +53,7 @@
           </DropDownButton.Styles>
           <DropDownButton.Flyout>
             <Flyout FlyoutPresenterClasses="nopadding"
-                    Placement="Top">
+                    Placement="{DynamicResource ColorPickerFlyoutPlacement}">
 
               <!-- The following is copy-pasted from the ColorView's control template.
                    It MUST always be kept in sync with the ColorView (which is master).

src/Avalonia.Controls.ColorPicker/Themes/Simple/ColorPicker.xaml

@@ -1,8 +1,10 @@
-<ResourceDictionary xmlns="https://github.com/avaloniaui"
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:controls="using:Avalonia.Controls"
                     xmlns:primitives="using:Avalonia.Controls.Primitives"
                     x:ClassModifier="internal">
+  
+  <PlacementMode x:Key="ColorPickerFlyoutPlacement">Top</PlacementMode>
 
   <ControlTheme x:Key="{x:Type ColorPicker}"
                 TargetType="ColorPicker">
@@ -50,7 +52,8 @@
             </Style>
           </DropDownButton.Styles>
           <DropDownButton.Flyout>
-            <Flyout Placement="Top">
+            <Flyout FlyoutPresenterClasses="nopadding"
+                    Placement="{DynamicResource ColorPickerFlyoutPlacement}">
 
               <!-- The following is copy-pasted from the ColorView's control template.
                    It MUST always be kept in sync with the ColorView (which is master).

src/Avalonia.Controls/Flyouts/PopupFlyoutBase.cs

@@ -47,6 +47,12 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<IInputElement?> OverlayInputPassThroughElementProperty =
             Popup.OverlayInputPassThroughElementProperty.AddOwner<PopupFlyoutBase>();
         
+        /// <summary>
+        /// Defines the <see cref="PlacementConstraintAdjustment"/> property
+        /// </summary>
+        public static readonly StyledProperty<PopupPositionerConstraintAdjustment> PlacementConstraintAdjustmentProperty =
+            Popup.PlacementConstraintAdjustmentProperty.AddOwner<PopupFlyoutBase>();
+        
         private readonly Lazy<Popup> _popupLazy;
         private Rect? _enlargedPopupRect;
         private PixelRect? _enlargePopupRectScreenPixelRect;
@@ -119,6 +125,13 @@ namespace Avalonia.Controls.Primitives
             set => SetValue(OverlayInputPassThroughElementProperty, value);
         }
 
+        /// <inheritdoc cref="Popup.PlacementConstraintAdjustment"/>
+        public PopupPositionerConstraintAdjustment PlacementConstraintAdjustment
+        {
+            get => GetValue(PlacementConstraintAdjustmentProperty);
+            set => SetValue(PlacementConstraintAdjustmentProperty, value);
+        }
+        
         IPopupHost? IPopupHostProvider.PopupHost => Popup?.Host;
 
         event Action<IPopupHost?>? IPopupHostProvider.PopupHostChanged
@@ -424,9 +437,7 @@ namespace Avalonia.Controls.Primitives
             else
             {
                 Popup.Placement = Placement;
-                Popup.PlacementConstraintAdjustment =
-                    PopupPositioning.PopupPositionerConstraintAdjustment.SlideX |
-                    PopupPositioning.PopupPositionerConstraintAdjustment.SlideY;
+                Popup.PlacementConstraintAdjustment = PlacementConstraintAdjustment;
             }
         }