Prevent WPF control from expanding beyond viewable area

If the previous answer doesn't fix the problem, you could also try binding the Width, Height of your grid to the ActualWidth, ActualHeight of your parent UserControl. Something like:

<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Class="WpfApplication.UserControl1"
x:Name="UserControl">
<Grid Height="{Binding ElementName=UserControl, Path=ActualHeight}"
      Width="{Binding ElementName=UserControl, Path=ActualWidth}" />

In this case you aren't setting an explicit width and height but you are limiting the Grids width/height to the constraints of the UserControl it sits in.

If you remove the Height from your GroupBox (which, as far as I understand, is what you want to do), then it will fill its container, unless there's a panel upstream that imposes its own sizing rules.

I used this simplified version of your XAML. I removed the template and the binding, and hard-coded some items, to make this stand alone; those changes won't affect the way layout is done.

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
    <GroupBox FontWeight="Bold" Header="Tables" Padding="2">
        <ScrollViewer>
            <ItemsControl FontWeight="Normal">
                <TextBlock>Foo</TextBlock>
                <TextBlock>Bar</TextBlock>
                <TextBlock>Baz</TextBlock>
            </ItemsControl>
        </ScrollViewer>
    </GroupBox>
</Window>

Run it, and you'll see that the content does indeed size to fit the window, and the scrollbar only enables when the window gets too small to see all three items. I believe this is what you want.

So the problem is most likely one of the parent panels, one you're not showing in your sample XAML. The problem you describe could occur if your GroupBox appears inside a StackPanel:

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
    <StackPanel>
        <GroupBox FontWeight="Bold" Header="Tables" Padding="2">
            <ScrollViewer>
                <ItemsControl FontWeight="Normal">
                    <TextBlock>Foo</TextBlock>
                    <TextBlock>Bar</TextBlock>
                    <TextBlock>Baz</TextBlock>
                </ItemsControl>
            </ScrollViewer>
        </GroupBox>
    </StackPanel>
</Window>

Now the GroupBox appears at the top of the Window, sized to exactly fit its contents. If you shrink the Window enough, the GroupBox will be cut off -- because it's sized to fit its content, not its container. This sounds like the problem you're describing.

The reason is that StackPanel asks its children what their ideal height is (based on their content), and uses that height. Without StackPanel (or something similar), the default is to respect the control's VerticalAlignment, and if that's set to the default value of Stretch, then the control is stretched to fill its parent. This means it won't be taller than its parent, which sounds like what you want.

Solution: remove the StackPanel (or whatever else is causing you problems) and use something else. Depending on what you're trying to accomplish, you might have better luck with a DockPanel or a Grid. Hard to tell without knowing more about your layout.

Edit: Okay, it looks like the problem is indeed the HeaderedContentControl parent -- but not directly. HeaderedContentControl isn't a panel, so it doesn't do any layout of its own (and its descendant, GroupBox, doesn't have this same problem). The problem is its default template -- which includes a StackPanel. The good news is, you're free to use a different template, let's say one with a DockPanel instead:

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <HeaderedContentControl>
    <HeaderedContentControl.Style>
      <Style TargetType="{x:Type HeaderedContentControl}">
        <Setter Property="Template">
          <Setter.Value>
            <ControlTemplate TargetType="{x:Type HeaderedContentControl}">
              <DockPanel>
                <ContentPresenter ContentSource="Header" DockPanel.Dock="Top"/>
                <ContentPresenter/>
              </DockPanel>
            </ControlTemplate>
          </Setter.Value>
        </Setter>
      </Style>
    </HeaderedContentControl.Style>
    <GroupBox FontWeight="Bold" Header="Tables" Padding="2">
      <ScrollViewer>
        <ItemsControl FontWeight="Normal">
          <TextBlock>Foo</TextBlock>
          <TextBlock>Bar</TextBlock>
          <TextBlock>Baz</TextBlock>
        </ItemsControl>
      </ScrollViewer>
    </GroupBox>
  </HeaderedContentControl>
</Window>

If you leave off the <HeaderedContentControl.Style> part, this reproduces your problem; but with the style in place, it allows the GroupBox to fill its container, so the ScrollViewer will get a scrollbar when you want it to.