As of 2016.1.217 for WPF, this issue affects both RadRibbonSplitButton and RadRibbonDropDown components.

If the designer is set to single frame mode, having either of these controls selected in the designer prevents opening of the XAML tab.  It immediately jumps back to the Design tab.  In split view, clicking an xml node for either of these components immediately shifts focus to the designer and the user needs to click a second time in the xml node to edit.

Not a major breaking issue, but can be inconvenient.  When scrolling through text with the arrow keys, once you get to one of these controls in the XML, it shifts focus to the designer and then the arrow keys just start changing the margins instead of jumping through lines of text.