Hi Steve,
It was fixed in the past but when we restructured RibbonWindow again recently to use WindowChrome, that issue was introduced again. Allowing custom chrome rendering (sometimes over non-client areas of windows) while still allowing glass support is a bit of a nightmare to implement due to bugs in the core Windows API. While our reimplementation is much cleaner than before and solves some issues with the old RibbonWindow, it did introduce this issue. We have put some time into it in the past but the normal workaround for this triggers black glass borders to display in certain circumstances. So far we haven't been able to solve that.