Why are relative bindings in a virtualized data grid sometimes not resolved?

Question

I am occasionally getting binding errors from an element in a cell of a data grid. The problematic binding has a relative source. The appearance of the error seems to be dependent on the number columns and on virtualization.

I've recreated a similar problem in a simplfied project. (It's a bit contrived, but it demonstrates the problem.)

Here is the window when it first opens:

Here is the window when it is resized (note the blank cells):

Here is the xaml:

<Window x:Class="Test.DataGridVirtualizationBindingTest"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:test="clr-namespace:Test"
        Title="DataGridVirtualizationBindingTest" Height="300" Width="300">

    <Window.Resources>
        <DataTemplate x:Key="IsReadyTemplate">
            <TextBox 
                Text="{Binding Path=DataContext.IsReady, RelativeSource={RelativeSource AncestorType={x:Type test:DataGridVirtualizationBindingTest}}}"
                />
        </DataTemplate>

    </Window.Resources>
    <Grid>
        <DataGrid 
            ItemsSource="{Binding Records}"
            AutoGenerateColumns="False"
            ScrollViewer.IsDeferredScrollingEnabled="True"
            VirtualizingPanel.IsVirtualizingWhenGrouping="True"
            VirtualizingPanel.VirtualizationMode="Recycling"
            EnableRowVirtualization="True"
            EnableColumnVirtualization="True"
            >

            <DataGrid.Columns>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
                <DataGridTemplateColumn Header="Ready" CellTemplate="{StaticResource IsReadyTemplate}"/>
            </DataGrid.Columns>
        </DataGrid>

    </Grid>
</Window>

The window's data context is an instance of this class:

class DataGridVirtualizationBindingTestVm 
{
    public DataGridVirtualizationBindingTestVm()
    {
        Records = new ObservableCollection<Record>(Enumerable.Range(1, 4000).Select(i => new Record { Field1 = i.ToString() }));

        IsReady = true;
    }

    public ObservableCollection<Record> Records { get; private set; }

    public bool IsReady { get; set; }
}

public class Record 
{
    public string Field1 { get; set; }
}

The error is

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='Test.DataGridVirtualizationBindingTest', AncestorLevel='1''. BindingExpression:Path=DataContext.IsReady; DataItem=null; target element is 'TextBox' (Name=''); target property is 'Text' (type 'String')

It doesn't occur for every cell. The framework is 4.5.2.

Why is the binding sometimes not resolved?

Answer 1

Simply remove

EnableColumnVirtualization="True"

Normally you don't need that (only if you have dozens of columns shown at once, but this is not something good for user experience - having long horizontal scroll) and it seems to be the case of the problem.