WPF CheckBox is not squared

I have a strange bug here. CheckBox controls has always been squared. Now I've noticed that it's not. It's one pixel wider than tall. After experimenting a bit with empty WPF applications, I've discovered that it depends on the UseLayoutRounding setting. If it's set to false, the CheckBox is never a square. If it's set to true, it depends on the actual layout values and how they are rounded, or snapped to pixels.

Just create an empty WPF application targeting .NET Framework 4.5.2, 4.6 or 4.6.1 (I've tested these) and run it on Windows 10. Here's the minimal XAML to reproduce. The third state shows another filled "square" box that makes it easier to recognise the form without measuring pixels.

<Window
    x:Class="CheckBoxTest.MainWindow"
    xmlns="http://ift.tt/o66D3f"
    xmlns:x="http://ift.tt/mPTqtT"
    Title="MainWindow" Height="350" Width="525" UseLayoutRounding="False">
    <Grid>
        <CheckBox HorizontalAlignment="Center" VerticalAlignment="Center" IsThreeState="True" IsChecked="{x:Null}"/>
    </Grid>
</Window>

Then drag the window's width and watch the CheckBox as it toggles between square and one pixel wider. In another of my applications it's always one pixel wider and never has a squared form.

Is that a bug in WPF? Does it only happen on Windows 10?

PS: Better not use a high DPI monitor for this, it may be harder to see there.