GridSplitter does not work correctly, expands child outside window

• .NET Core Version: 3.0 Preview 8

• Windows version: Windows 10

• Does the bug reproduce also in WPF for .NET Framework 4.8?: Not sure

Problem description:

I have a window with a grid with 2 rows. Between the rows I have a grid splitter. I want the bottom row to start with an initial height, but allow user to expand it. Both rows can collapse to height 0 or expand to fill the entire window.

When I drag the grid splitter to the top edge and continue dragging the mouse (without releasing the button) the bottom row keeps expanding for ever. I expect the gird splitter to stop expanding the bottom row as soon as the splitter hits the top of the window.

When I drag the grid splitter down it works correctly.

Actual behavior: When I drag the grid splitter to the top edge and continue dragging the mouse (without releasing the button) the bottom row keeps expanding for ever.

Expected behavior: I expect the gird splitter to stop expanding the bottom row as soon as the splitter hits the top of the window.

Minimal repro: Create a new WPF app with this xaml:

<Window x:Class="WpfSplitterApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfSplitterApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="60" />
        </Grid.RowDefinitions>

        <Grid Grid.Row="0">
            <TextBox x:Name="tb1" AcceptsReturn="True" Margin="10" Background="LightCyan" VerticalScrollBarVisibility="Visible"
                     Text="{Binding ActualHeight, Mode=OneWay, RelativeSource={RelativeSource Self}}"/>
        </Grid>

        <GridSplitter Grid.Row="0" HorizontalAlignment="Stretch" VerticalAlignment="Bottom" Height="6"/>
        
        <Grid Grid.Row="1">
            <TextBox x:Name="tb2" AcceptsReturn="True" Margin="10" Background="LightGreen" VerticalScrollBarVisibility="Visible"
                     Text="{Binding ActualHeight, Mode=OneWay, RelativeSource={RelativeSource Self}}"/>
        </Grid>
    </Grid>
</Window> 

Run the application and move/size the application window so you can test dragging the grid splitter from its location to a point above the window. Expand the grid splitter up and continue beyond window. The height is displayed in the text box. You will see it increasing.