Why would a CheckBox in a GroupBox header not properly respond to hit tests?
===

Ok, simple thing.  I have a GroupBox of which I want to add a CheckBox as its heading.  That way the user can toggle on or off that option.  Simple enough.

However, the hit-testing of the `CheckBox` is all wonky and doesn't always respond to being clicked.  Any reason you can think of why?  I've tried direct content (below), explicit `TextBlock` (also below), I've tried setting the both the `TextBlock` and `GroupBox`'s `Background` properties to transparent (null won't record hit tests), I've tried disabling hit testing on the `TextBlock`, but nothing seems to work.  There seems to be some dead areas when you click around with the mouse.  Try it!

This causes the issue...

    <GroupBox
        Margin="0,4,0,0"
        Padding="8">

        <GroupBox.Header>
            <!-- Try inline content -->
            <CheckBox x:Name="chkFindReplaceEnabled" Content="Enable Find/Replace during copy:" />
        </GroupBox.Header>

        <!-- Bla bla -->

    </GroupBox>

As does this...

    <GroupBox
        Margin="0,4,0,0"
        Padding="8">

        <GroupBox.Header>

            <CheckBox x:Name="chkFindReplaceEnabled"
                Background="Transparent">

                <TextBlock Text="Enable Find/Replace during copy:"
                    IsHitTestVisible="True" <!-- Explicitly enable hit-testing -->
                    Background="Transparent" />

            </CheckBox>

        </GroupBox.Header>

        <!-- Bla bla -->

    </GroupBox>

And this...

    <GroupBox
        Margin="0,4,0,0"
        Padding="8">

        <GroupBox.Header>

            <CheckBox x:Name="chkFindReplaceEnabled"
                Background="Transparent">

                <TextBlock Text="Enable Find/Replace during copy:"
                    IsHitTestVisible="False" /> <!-- Explicitly disable hit-testing -->

            </CheckBox>

        </GroupBox.Header>

        <!-- Bla bla -->

    </GroupBox>

Thoughts on why this happens or how I can fix it?