Windows 10 has a feature to create multiple virtual desktops. When a ribbon application is opened which uses the metro theme, the shadow (or outer glow) is visible on all virtual desktops. So when you switch to a different desktop the application is hidden but the shadow is still visible. And when your cursor reaches the border, it allows to resize the application. When you click the border, you automatically switch to the desktop where the application is opened in.
Disabling the shadow probably solves the issue, but I would rather have my shadow back...
Best regards, Tobias Lingemann.