Silverlight Windows Phone 7 - Button click event lost after using hardware back button

On a Windows phone 7 Silverlight project, when collapsing a panel containing a button, and then using the hardware back button to return (setting again visibility to visible), the FIRST click event is lost.

This has been driving me crazy for a while.

Here is an easy way to reproduce, start a new Wp7 silverlight app, and on xaml add two content panels with a button.

    <Grid x:Name="ContentPanel" Grid.Row="1" Visibility="Visible">
        <Button Content="Change Visibility" Click="ChangeButton_Click"/>
    </Grid>
    <Grid x:Name="ContentPanel2" Grid.Row="1" Visibility="Collapsed">
        <Button Content="Back" Click="BackButton_Click"/>
    </Grid>

The code behind has handlers for both buttons and for the hardware back button to simply switch the visibility between the two panels.

    public MainPage()
    {
        InitializeComponent();
        BackKeyPress += new EventHandler<System.ComponentModel.CancelEventArgs>(MainPage_BackKeyPress);
    }

    void MainPage_BackKeyPress(object sender, System.ComponentModel.CancelEventArgs e)
    {
        if (ContentPanel2.Visibility == Visibility.Visible)
        {
            ContentPanel.Visibility = Visibility.Visible;
            ContentPanel2.Visibility = Visibility.Collapsed;
            e.Cancel = true;
        }
    }

    private void ChangeButton_Click(object sender, RoutedEventArgs e)
    {
        ContentPanel.Visibility = Visibility.Collapsed;
        ContentPanel2.Visibility = Visibility.Visible;
    }

    private void BackButton_Click(object sender, RoutedEventArgs e)
    {
        ContentPanel.Visibility = Visibility.Visible;
        ContentPanel2.Visibility = Visibility.Collapsed;
    }

And now the weird thing:

• If you use the on-screen back button switching works开发者_StackOverflow fine
• If you use the hardware back button to switch back, then the FIRST click event afterwards will be lost!

This happens both on emulator and on a real device.

Is this a bug? or what am I doing wrong?

Someone called Fendors (Thanks!) replied to my question on the app hub forums. He didn't find the reason why it happened, but found a workaround:

"I didn't figure out fully what is happening when the hardware back button is clicked, but did find a workaround. I was initially thinking maybe there was some sort of focus issue and found that if you set the content panel from visible to collapsed to visible, it puts it into a good state"

void MainPage_BackKeyPress(object sender, System.ComponentModel.CancelEventArgs e) 
        { 
            if (ContentPanel2.Visibility == Visibility.Visible) 
            { 
                ContentPanel2.Visibility = Visibility.Collapsed; 
                ContentPanel.Visibility = Visibility.Visible; 
                ContentPanel.Visibility = Visibility.Collapsed; 
                ContentPanel.Visibility = Visibility.Visible; 

                e.Cancel = true; 
            } 
        } 

Ran into this same problem. Seems like a bug (great find on the workaround). I wrote this simple extension method for the workaround:

public static class UIElementExtensions
{
    public static void ShowForReal(this UIElement element)
    {
        element.Visibility = Visibility.Visible;
        element.Visibility = Visibility.Collapsed;
        element.Visibility = Visibility.Visible;
    }
}

then just call ContentPanel.ShowForReal();