using System;

using System.Collections.Generic;

using System.Text;

using System.Windows.Controls;

using System.Windows;

using System.Windows.Media;

using System.Windows.Media.Animation;

namespace FadePanel

{

 class FadeStackPanel:StackPanel

 {

 DoubleAnimation fadeOut = new DoubleAnimation(1, 0, new Duration(TimeSpan.FromMilliseconds(1000)));

 DoubleAnimation fadeIn = new DoubleAnimation(0, 1, new Duration(TimeSpan.FromMilliseconds(1000)));

 public FadeStackPanel ():base()

 {

 }

 protected override void OnRenderSizeChanged (System.Windows.SizeChangedInfo sizeInfo)

 {

 base.OnRenderSizeChanged(sizeInfo);

 int totalChildren = base.VisualChildrenCount;

 //Enumerates stackpanel children

 for (int i = 0; i < totalChildren; i++)

 {

 UIElement child = base.Children[i];

 FrameworkElement elem=child as FrameworkElement;

 if (elem != null)

 {

 bool outside = (child.DesiredSize.Width < child.RenderSize.Width);

 if (outside)

 {

 if (elem.Tag == null) elem.BeginAnimation(UIElement.OpacityProperty, fadeOut);

 elem.Tag = 0;

 }

 else

 if (elem.Tag != null)

 {

 child.BeginAnimation(UIElement.OpacityProperty, fadeIn);

 elem.Tag = null;

 }

 }

 }

 }

 }

}

<Window

 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

 xmlns:l="clr-namespace:FadePanel"

 x:Class="FadePanel.Window1"x:Name="Window" Title="Window1"Width="658" Height="480" Background="Cyan">

 <Grid x:Name="LayoutRoot">

 <l:FadeStackPanel Margin="10,10,28,22" x:Name="fpanel">

 <TextBox Width="534" Height="31.5" Text="TextBox" TextWrapping="Wrap"/>

 <TextBox Width="261" Height="42" Text="TextBox" TextWrapping="Wrap"/>

 </l:FadeStackPanel>

 </Grid>

</Window>