The designer of VS crashes with a "Key cannot be null" when custom markup extension is used in Style Setter. This reproduces in the sample app attached in the blog (http://www.telerik.com/blogs/bring-office-2016-inspired-style-to-your-app-with-ui-for-wpf) on the TextBlock of the RabRadioButton HeadingStyle (snapshot attached) if you open MainWindow in Designer. Runtime everything works as expected.

In a two monitors scenario the ApplicationMenu appears on the left monitor when the RibbonView with negative left margin is hosted in a RibbonWindow which is maximized on the right monitor.

EDIT:
We are closing this issue as the described behavior as expected. When logged first, the negative left margin of RibbonView was not taken into account.
When popup opens and its parent starts on the left monitor, popup calculates that it cannot fit entirely and changes its direction of opening. This expected by the WPF Framework.
When you give negative left margin of the RibbonView, its application button might also goes with 1-2 (or more) pixels on the left monitor. Then popup (the application menu opens in a popup) has no space and changes its opening direction to left.
This is expected and the only reasonable solution would be changing the negative left margin to positive or 0.