Enable Windows 8.1+ Per-Monitor DPI support for Desktop WPF Apps. Instead of attempting to understand this long MSDN article, take advantage of my personal suffering and use this instead:
Install-Package PerMonitorDpiBehavior<i:Interaction.Behaviors>
<perMonitorDpi:PerMonitorDpiBehavior/>
</i:Interaction.Behaviors>To observe the difference, attach a normal monitor to a Surface Pro 2 or other Retina-DPI monitor, then move your window between the two monitors. Per-Monitor DPI apps will stay sharp, normal apps will have blurred text on the Retina monitor.