Make DropDownButton public. This is required for AvalonEdit to work in .NET 3.5.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Search/DropDownButton.cs

@@ -12,17 +12,26 @@ namespace ICSharpCode.AvalonEdit.Search
 	/// <summary>
 	/// A button that opens a drop-down menu when clicked.
 	/// </summary>
-	class DropDownButton : ButtonBase
+	public class DropDownButton : ButtonBase
 	{
+		/// <summary>
+		/// Identifies the <see cref="DropDownContent" /> dependency property.
+		/// </summary>
 		public static readonly DependencyProperty DropDownContentProperty
 			= DependencyProperty.Register("DropDownContent", typeof(Popup),
 			                              typeof(DropDownButton), new FrameworkPropertyMetadata(null));
 		
+		/// <summary>
+		/// The key that identifies the <see cref="IsDropDownContentOpen" /> dependency property.
+		/// </summary>
 		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Security", "CA2104:DoNotDeclareReadOnlyMutableReferenceTypes")]
 		protected static readonly DependencyPropertyKey IsDropDownContentOpenPropertyKey
 			= DependencyProperty.RegisterReadOnly("IsDropDownContentOpen", typeof(bool),
 			                                      typeof(DropDownButton), new FrameworkPropertyMetadata(false));
 		
+		/// <summary>
+		/// Identifies the <see cref="IsDropDownContentOpen" /> dependency property.
+		/// </summary>
 		public static readonly DependencyProperty IsDropDownContentOpenProperty = IsDropDownContentOpenPropertyKey.DependencyProperty;
 		
 		static DropDownButton()
@@ -30,16 +39,23 @@ namespace ICSharpCode.AvalonEdit.Search
 			DefaultStyleKeyProperty.OverrideMetadata(typeof(DropDownButton), new FrameworkPropertyMetadata(typeof(DropDownButton)));
 		}
 		
+		/// <summary>
+		/// Gets/Sets the popup that is used as drop-down content.
+		/// </summary>
 		public Popup DropDownContent {
 			get { return (Popup)GetValue(DropDownContentProperty); }
 			set { SetValue(DropDownContentProperty, value); }
 		}
 		
+		/// <summary>
+		/// Gets whether the drop-down is opened.
+		/// </summary>
 		public bool IsDropDownContentOpen {
 			get { return (bool)GetValue(IsDropDownContentOpenProperty); }
 			protected set { SetValue(IsDropDownContentOpenPropertyKey, value); }
 		}
 		
+		/// <inheritdoc/>
 		protected override void OnClick()
 		{
 			if (DropDownContent != null && !IsDropDownContentOpen) {