The following XAML produces a window with strange behavior around the textbox:

<Window x:Class="WpfSandbox.CuriousExample"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="CuriousExample" Height="300" Width="300">
    <DockPanel Margin="15">
        <TextBox BorderThickness="1" BorderBrush="#FF000000"></TextBox>
    </DockPanel>
</Window>

What happens, at least during my limited testing, is that the textbox renders with an inset border pattern (top/left is black, right/bottom is grey). However, when you resize to any position except the original, the entire textbox border goes to black. Whenever you return the window to the exact number of on-screen pixels the form had when it first loaded, it's inset again.

I'm guessing it isn't pixel snapping as I can easily correct the problem with this code:

<Window x:Class="WpfSandbox.CuriousExample"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="CuriousExample" Height="300" Width="300">
    <DockPanel Margin="15">
        <Border BorderThickness="1" BorderBrush="#FF000000">
            <TextBox BorderThickness="0" ></TextBox>
        </Border>
    </DockPanel>
</Window>

Anyone care to venture an explanation as to what I'm seeing? Or is it all in my head?

Like I said, the above workaround can resolve this problem - just trying to understand what is happening here.

Thanks,

-Scott