If you set the AllowsTransparency
property on the Window
(even without setting any transparency values) the border disappears and you can only resize via the grip.
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="640" Height="480"
WindowStyle="None"
AllowsTransparency="True"
ResizeMode="CanResizeWithGrip">
<!-- Content -->
</Window>
Result looks like: