Silverlight: Layout changes when I call NavigationService.Navigate

Question

This is a really weird bug. I have no idea why it could be happening. I know that posting it here is a bit of a long-shot, but I'm out of other ideas.

I have two ListBoxs that act as menus.

                <ListBox Margin="56,8,15,0" FontSize="64"
                         ItemsSource="{Binding FavoriteSections}"
                         SelectionChanged="MenuList_SelectionChanged">
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel>
                                <toolkit:ContextMenuService.ContextMenu>
                                    <toolkit:ContextMenu>
                                        <toolkit:MenuItem Header="Remove" Click="FavoritesContextMenuItem_Click" />
                                    </toolkit:ContextMenu>
                                </toolkit:ContextMenuService.ContextMenu>

                                <TextBlock Text="{Binding DisplayName}" />
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>


                <ListBox x:Name="sectionList" Margin="56,8,15,0" FontSize="64" 
                         SelectionChanged="MenuList_SelectionChanged"
                         ItemsSource="{Binding SectionViewModels}">
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel>
                                <toolkit:ContextMenuService.ContextMenu>
                                    <toolkit:ContextMenu>
                                        <toolkit:MenuItem Header="Add to favorites" Click="SectionContextMenuItem_Click" />
                                    </toolkit:ContextMenu>
                                </toolkit:ContextMenuService.ContextMenu>
                                <TextBlock Text="{Binding DisplayName}" />
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>

The bug exists across both of them.

When the selection changes on either menu, this method is called:

    void MenuList_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (e.AddedItems.Count == 0)
        {
            return;
        }

        Uri page = null;
        object selected = e.AddedItems[0];
        if (selected is NavigableItem)
        {
            NavigableItem selectedItem = (NavigableItem)selected;
            page = selectedItem.Page;
        }
        else if (selected is SectionViewModel)
        {
            SectionViewModel selectedVM = (SectionViewModel)selected;
            page = selectedVM.Section.Page;
        }

        Debug.Assert(page != null, "What is the type of `selected`?");

        // if I comment out this line, the problem goes away:
        NavigationService.Navigate(page);

        ListBox selectedBox = (ListBox)sender;
        selectedBox.SelectedIndex = -1;
    }

If I comment out the NavigationService.Navigate() line, the problem goes away. If I replace the line with a different URI, the problem remains.

About 70% of the time, when I click on a menu item, the content jumps all over the page. (The remaining 30%, no bug occurs.) It happens too quickly to see what's really going on, but different UI elements overlap each other.

This only occurs the first time I click on something in those menus during the app's lifetime. If I hit "back" then select a menu item again, the problem will not occur.

What could be happening here? I really have no idea. The code-behind doesn't have a OnNavigatedFrom method, so I don't think it's a problem there.

I'm using Silverlight for Windows Phone 7

Update: Mysteriously, I can't seem to reproduce this in the debugger - only after deploying the app and running it in the emulator unattached. ???

Update 2: The bug appears when NavigationService.Navigate() is called from the Click event handler of a button, as well:

<Button Content="Foo" Click="Button_Click" Grid.Row="0"/>

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            NavigationService.Navigate(new Uri("/Views/sections.xaml?section=43", UriKind.Relative));
        }

Looks like the bug has to do with the navigation, not the UI element used to trigger the call.

Update 3: More weirdness. Still not able to reproduce the app while the debugger is attached. If I make the loading progress bar always collapsed, the bug disappears:

                <ProgressBar x:Name="LoadingProgressBar"
                 IsIndeterminate="True"
                 Visibility="Collapsed"
                 Style="{StaticResource PerformanceProgressBar}"
                 VerticalAlignment="Top"/>

Alternatively, commenting out this line in code-behind makes the bug disappear:

LoadingProgressBar.Visibility = Visibility.Collapsed;

I really don't understand what's going on here. That line of code is not executed when the page is navigated from.

Here is the full XAML of the control that's getting messed up:

                    <ProgressBar x:Name="LoadingProgressBar"
                     IsIndeterminate="True"
                     Visibility="Collapsed"
                     Style="{StaticResource PerformanceProgressBar}"
                     VerticalAlignment="Top"/>

                <TextBlock x:Name="DownloadFailed"
                         Visibility="Collapsed"
                         Style="{StaticResource disabledText}"
                         Margin="56,8,8,-8" >
                    FooBar.com could not be reached. Do you have a network connection?
                </TextBlock>

                <ListBox x:Name="sectionList" Margin="56,8,15,0" FontSize="64" 
                         SelectionChanged="MenuList_SelectionChanged"
                         ItemsSource="{Binding SectionViewModels}">
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel>
                                <toolkit:ContextMenuService.ContextMenu>
                                    <toolkit:ContextMenu>
                                        <toolkit:MenuItem Header="Add to favorites" Click="SectionContextMenuItem_Click" />
                                    </toolkit:ContextMenu>
                                </toolkit:ContextMenuService.ContextMenu>
                                <TextBlock Text="{Binding DisplayName}" />
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>

            </Grid>
        </controls:PivotItem>

Answer 1

The problem lies in your usage of the Indeterminate ProgressBar. All its animations are done on the UI thread, and not the Compositor thread, as is the usual practise. Since you are already using the Windows Phone Toolkit, you can easily replace your ProgressBar with the PerformanceProgressBar offered by the toolkit. This should solve your problems.