ListDetailsView navigation data loss

[huwpascoe]

Describe the bug

MasterDetailsView, navigation events are processed in incorrect order resulting in data loss for controls that use {x:Bind UpdateSourceTrigger=LostFocus} type binding.

Steps to Reproduce

    public class MyItem
    {
        public string MyName { get; set; }
        public string MyValue { get; set; }
    }

    public class ViewModel
    {
        public ObservableCollection<MyItem> Items { get; set; } = new ObservableCollection<MyItem>()
        {
            new MyItem(){ MyName = "Alice" },
            new MyItem(){ MyName = "Bob" },
        };
    }

    <Page.Resources>
        <DataTemplate x:Key="ListTemplate" x:DataType="local:MyItem">
            <TextBlock Text="{x:Bind MyName}" />
        </DataTemplate>
        <DataTemplate x:Key="DetailsTemplate" x:DataType="local:MyItem">
            <StackPanel Margin="16">
                <TextBlock Text="{x:Bind MyName}" FontSize="32" />
                <TextBox Header="Value" Text="{x:Bind MyValue, Mode=TwoWay}" />
                <TextBox Header="Dummy" />
            </StackPanel>
        </DataTemplate>
    </Page.Resources>
    <Grid>
        <controls:MasterDetailsView
          ItemsSource="{x:Bind ViewModel.Items}"
          ItemTemplate="{StaticResource ListTemplate}"
          DetailsTemplate="{StaticResource DetailsTemplate}"
          NoSelectionContent="Select an item" />
    </Grid>

Expected behavior

MasterDetailsView
1 ItemSelected
2 LostFocus => {x:Bind} saves data
3 Navigate

Actual Behavior

1 ItemSelected
2 Navigate => unsaved data overwritten
3 LostFocus

Screenshots

Environment

NuGet Package(s): 
Microsoft.NETCore.UniversalWindowsPlatform
Microsoft.Toolkit.Uwp.UI.Controls

Package Version(s): 
6.2.10
6.1.1

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [x] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [x] 2019 (version: 16.7.5) 
- [ ] 2019 Preview (version: )

[ghost]

Hello huwpascoe, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌

[Kyaa-dost]

@huwpascoe Thanks for highlighting the issue. Do you want to work on it and create a PR?

[michael-hawker]

Yeah, tested with a WinUI TabView and that works as expected. I imagine it has something to do with how we update the selected item and update the presenter from the selection somewhere here?

https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/90871947640ccc1622ca9328c2c78aae27a241e7/Microsoft.Toolkit.Uwp.UI.Controls/MasterDetailsView/MasterDetailsView.cs#L106-L142

[Rosuavio]

Discovered this issue, could be related #4244. I am exploring a solution to both in my branch. #4222

[Rosuavio]

@michael-hawker I don't believe that #4255 fixed that issue. #4222 correctly tests for the underlying cause of this issues but lacks a solution.