Added DropDown.MaxDropDownHeight.

Fixes #654.

src/Avalonia.Controls/DropDown.cs

@@ -28,6 +28,12 @@ namespace Avalonia.Controls
                 o => o.IsDropDownOpen,
                 (o, v) => o.IsDropDownOpen = v);
 
+        /// <summary>
+        /// Defines the <see cref="MaxDropDownHeight"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> MaxDropDownHeightProperty =
+            AvaloniaProperty.Register<DropDown, double>(nameof(MaxDropDownHeight), 200);
+
         /// <summary>
         /// Defines the <see cref="SelectionBoxItem"/> property.
         /// </summary>
@@ -56,6 +62,15 @@ namespace Avalonia.Controls
             set { SetAndRaise(IsDropDownOpenProperty, ref _isDropDownOpen, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the maximum height for the dropdown list.
+        /// </summary>
+        public double MaxDropDownHeight
+        {
+            get { return GetValue(MaxDropDownHeightProperty); }
+            set { SetValue(MaxDropDownHeightProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the item to display as the control's content.
         /// </summary>

src/Avalonia.Themes.Default/DropDown.xaml

@@ -30,16 +30,19 @@
                     Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z"/>
             </ToggleButton>
             <Popup Name="PART_Popup"
-                   IsOpen="{TemplateBinding Path=IsDropDownOpen, Mode=TwoWay}"
-                   MinWidth="{TemplateBinding Path=Bounds.Width}"
+                   IsOpen="{TemplateBinding IsDropDownOpen, Mode=TwoWay}"
+                   MinWidth="{TemplateBinding Bounds.Width}"
+                   MaxHeight="{TemplateBinding MaxDropDownHeight}"
                    PlacementTarget="{TemplateBinding}"
                    StaysOpen="False">
               <Border BorderBrush="{StyleResource ThemeBorderMidBrush}"
                       BorderThickness="1">
-                <ItemsPresenter Name="PART_ItemsPresenter"
-                                Items="{TemplateBinding Items}" 
-                                ItemTemplate="{TemplateBinding ItemTemplate}"
-                                MemberSelector="{TemplateBinding MemberSelector}"/>
+                <ScrollViewer>
+                  <ItemsPresenter Name="PART_ItemsPresenter"
+                                  Items="{TemplateBinding Items}"
+                                  ItemTemplate="{TemplateBinding ItemTemplate}"
+                                  MemberSelector="{TemplateBinding MemberSelector}"/>
+                </ScrollViewer>
               </Border>
             </Popup>
           </Grid>