Grid height not adjusting properly when a row height with SharedSizeGroup changes

Question

I have two grids with three rows each. The first and last row of each grid has a fixed height while the middle rows have auto height and share their height using SharedSizeGroup.

First, the content of the right grid determines the height of the size group. When the content of the right grid shrinks so that the content of the left grid determines the height of the size group, the overall size of the left grid is not adjusted correctly.

See the following sample app. Click the increase button to increase the size of the textblock in the right grid. The size of the left grid changes accordingly. Now decrease the size. When the textblock becomes smaller than the textblock in the left grid, the total size of the left grid doesn't shrink. Is this a bug or am i missing something?

<StackPanel Orientation="Horizontal" Grid.IsSharedSizeScope="True">
    <StackPanel Orientation="Vertical" Width="100">
        <Grid Background="Green">
            <Grid.RowDefinitions>
                <RowDefinition Height="15" />
                <RowDefinition SharedSizeGroup="Group1" />
                <RowDefinition Height="15" />
            </Grid.RowDefinitions>
            <TextBlock Background="Blue" Grid.Row="0" />
            <TextBlock Background="Red" Grid.Row="1" Name="textBlock1" Height="100" />
            <TextBlock Background="Blue" Grid.Row="2" />
        </Grid>
        <TextBlock Text="{Binding Path=ActualHeight, ElementName=grid1}" />
    </StackPanel>
    <StackPanel Orientation="Vertical" Width="100">
        <Grid Background="Green">
            <Grid.RowDefinitions>
                <RowDefinition Height="15" />
                <RowDefinition SharedSizeGroup="Group1" />
                <RowDefinition Height="15" />
            </Grid.RowDefinitions>
            <TextBlock Background="Blue" Grid.Row="0" />
            <TextBlock Background="Red" Grid.Row="1" Name="textBlock2" Height="150" />
            <TextBlock Background="Blue" Grid.Row="2" />
        </Grid>
        <TextBlock Text="{Binding Path=ActualHeight, ElementName=grid2}" />
    </StackPanel>
    <Button Height="24" Click="Button_Click_1" VerticalAlignment="Top">Increase</Button>
    <Button Height="24" Click="Button_Click_2" VerticalAlignment="Top">Decrease</Button>
</StackPanel>

---

private void Button_Click_1(object sender, RoutedEventArgs e)
{
    textBlock2.Height += 30;
}

private void Button_Click_2(object sender, RoutedEventArgs e)
{
    textBlock2.Height -= 30;
}

Answer 1

The rows are staying the same height - the size of the second TextBlock is changing, while the size of the first TextBlock remains 100.

To demonstrate this, make the following changes:

• Add ShowGridLines="True" to each of your Grids

• Change your named TextBlocks to show their ActualHeight as their text:

      <TextBlock Background="Red" Grid.Row="1" Name="textBlock2" Height="150"
                 Text="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"/>
  

You will see that the SharedSizeGroup row will be the maximum ActualHeight of the two TextBlocks.

Update: A Short Project To Show What's Happening
I've created a quick-n-dirty project to show what's happening. It turns out that when the right grid gets smaller than the original size, the left grid does an Arrange but not a Measure. I am not sure I understand this completely - I have posted a follow-up question with this same solution.

Here is the solution that I created, based on your original. It simply wraps the basic grid in a custom class that spews out info (via event) when Measure and Arrange are called. In the main window, I just put that info into a list box.

InfoGrid and InfoGridEventArgs classes

using System.Windows;
using System.Windows.Controls;
namespace GridMeasureExample
{
    class InfoGrid : Grid
    {
        protected override Size ArrangeOverride(Size arrangeSize)
        {
            CallReportInfoEvent("Arrange");
            return base.ArrangeOverride(arrangeSize);
        }
        protected override Size MeasureOverride(Size constraint)
        {
            CallReportInfoEvent("Measure");
            return base.MeasureOverride(constraint);
        }
        public event EventHandler<InfoGridEventArgs> ReportInfo;
        private void CallReportInfoEvent(string message)
        {
            if (ReportInfo != null)
                ReportInfo(this, new InfoGridEventArgs(message));
        }
    }
    public class InfoGridEventArgs : EventArgs
    {
        private InfoGridEventArgs()
        {
        }
        public InfoGridEventArgs(string message)
        {
            this.TimeStamp = DateTime.Now;
            this.Message = message;
        }
        public DateTime TimeStamp
        {
            get;
            private set;
        }
        public String Message
        {
            get;
            private set;
        }
    }
}

Main Window XAML

<Window x:Class="GridMeasureExample.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"    
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:GridMeasureExample"
        Title="SharedSizeGroup" Height="500" Width="500">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <StackPanel Grid.Column="0" 
                    Grid.Row="0"
                    Orientation="Horizontal" 
                    HorizontalAlignment="Left"
                    VerticalAlignment="Top"
                    Grid.IsSharedSizeScope="True">

            <StackPanel Orientation="Vertical" Width="100">
                <local:InfoGrid x:Name="grid1" Background="Green" ShowGridLines="True">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="15" />
                        <RowDefinition SharedSizeGroup="Group1" />
                        <RowDefinition Height="15" />
                    </Grid.RowDefinitions>
                    <TextBlock Background="Blue" Grid.Row="0" Text="Row 0"/>
                    <TextBlock Background="Red" Grid.Row="1" Name="textBlock1" Height="100"
                           Text="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"/>
                    <TextBlock Background="Blue" Grid.Row="2" Text="Row 2" />
                </local:InfoGrid>
                <TextBlock Text="{Binding Path=ActualHeight, ElementName=grid1}" />
            </StackPanel>

            <StackPanel Orientation="Vertical" Width="100">
                <local:InfoGrid x:Name="grid2" Background="Yellow" ShowGridLines="True">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="15" />
                        <RowDefinition SharedSizeGroup="Group1" />
                        <RowDefinition Height="15" />
                    </Grid.RowDefinitions>
                    <TextBlock Background="Orange" Grid.Row="0" Text="Row 0" />
                    <TextBlock Background="Purple" Grid.Row="1" Name="textBlock2" Height="150"
                           Text="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"/>
                    <TextBlock Background="Orange" Grid.Row="2" Text="Row 2" />
                </local:InfoGrid>
                <TextBlock Text="{Binding Path=ActualHeight, ElementName=grid2}" />
            </StackPanel>

        </StackPanel>

        <ListBox x:Name="lstInfo"
                 Grid.Column="1"
                 Grid.Row="0"
                 Margin="10,0,0,0"
                 HorizontalAlignment="Stretch"
                 VerticalAlignment="Stretch" />

        <UniformGrid Grid.Column="0"
                     Grid.Row="1"
                     Grid.ColumnSpan="2"
                     Columns="2"
                     HorizontalAlignment="Center"
                     Margin="5">
            <Button x:Name="btnIncrease" Margin="4,0">Increase</Button>
            <Button x:Name="btnDecrease" Margin="4,0">Decrease</Button>
        </UniformGrid>

    </Grid>

</Window>

Main Window Constructor (only code in code-behind)

public Window1() { InitializeComponent();

    btnIncrease.Click += (s, e) => 
        {
            lstInfo.Items.Add(String.Format("{0} Increase Button Pressed", DateTime.Now.ToString("HH:mm:ss.ffff")));
            textBlock2.Height += 30;
        };
    btnDecrease.Click += (s, e) =>
        {
            lstInfo.Items.Add(String.Format("{0} Decrease Button Pressed", DateTime.Now.ToString("HH:mm:ss.ffff")));
            if (textBlock2.ActualHeight >= 30)
                textBlock2.Height -= 30;
        };

    grid1.ReportInfo += (s, e) => lstInfo.Items.Add(String.Format("{0} Left Grid: {1}", e.TimeStamp.ToString("HH:mm:ss.ffff"), e.Message));
    grid2.ReportInfo += (s, e) => lstInfo.Items.Add(String.Format("{0} Right Grid: {1}", e.TimeStamp.ToString("HH:mm:ss.ffff"), e.Message));
}