We were able to reproduce it with these settings:
- Windows 7 (doesn't happen in Windows 8+)
- Aero Basic (doesn't happen with Aero enabled)
- Second monitor attached (doesn't happen on primary monitor)
We spent a while on this today but were not able to find a good solution. The problem that appears is that when using the outer glow to resize the window in the specific scenario above, something goes wonky with the window rendering that seems rather low-level. it's almost like the double buffer maintained by WPF gets messed up.
We did find that if you set HasOuterGlow="False" on your WindowChrome instance, it will not run into the problem. The outer glow is a second Window that has AllowsTransparency="True" and sits behind the themed window, but only in Metro themes. It tracks the main themed window and resizes/moves with it. When the end user clicks on the outer glow, our code initiates a resize operation start on the main themed window instead. But something in Windows 7 (again only in the specific scenario above) seems to not like that having the AllowsTransparency window around for some reason. It's odd that it works fine on the primary monitor but not on the second. Our rendering code doesn't change based on monitor setup, so it seems like something in Microsoft's code is broken in that scenario.