Fix TextBlock constraint not getting initialized.

Perspex.Controls/Presenters/ScrollContentPresenter.cs

@@ -124,21 +124,25 @@ namespace Perspex.Controls.Presenters
             if (rect.Bottom > offset.Y + this.Viewport.Height)
             {
                 offset = offset.WithY(rect.Bottom - this.Viewport.Height);
+                e.Handled = true;
             }
 
             if (rect.Y < offset.Y)
             {
                 offset = offset.WithY(rect.Y);
+                e.Handled = true;
             }
 
             if (rect.Right > offset.X + this.Viewport.Width)
             {
                 offset = offset.WithX(rect.Right - this.Viewport.Width);
+                e.Handled = true;
             }
 
             if (rect.X < offset.X)
             {
                 offset = offset.WithX(rect.X);
+                e.Handled = true;
             }
 
             this.Offset = offset;

Perspex.Controls/Presenters/TextPresenter.cs

@@ -92,11 +92,6 @@ namespace Perspex.Controls
             set { this.SetValue(SelectionEndProperty, value); }
         }
 
-        public new FormattedText FormattedText
-        {
-            get { return base.FormattedText; }
-        }
-
         public int GetCaretIndex(Point point)
         {
             var hit = this.FormattedText.HitTestPoint(point);

Perspex.Controls/TextBlock.cs

@@ -35,6 +35,8 @@ namespace Perspex.Controls
 
         private FormattedText formattedText;
 
+        private Size constraint;
+
         public TextBlock()
         {
             Observable.Merge(
@@ -104,17 +106,20 @@ namespace Perspex.Controls
 
         protected virtual FormattedText CreateFormattedText()
         {
-            return new FormattedText(
+            var result = new FormattedText(
                 this.Text,
                 this.FontFamily,
                 this.FontSize,
                 this.FontStyle);
+            result.Constraint = constraint;
+            return result;
         }
 
         protected void InvalidateFormattedText()
         {
             if (this.formattedText != null)
             {
+                this.constraint = this.formattedText.Constraint;
                 this.formattedText.Dispose();
                 this.formattedText = null;
             }