When you select panels in a docklayoutmanager, the keyboard focus does not go to the inner content of the panels.
This was working in version 15.1.7.
Since I have the source code, the reason is that you've switched the LayoutPanel to using a new property called ContentPresenter. In version 15.1.7, you were using the Control property. Unfortunately, all your keyboard focus code is still passing in the Control property value as the root element to search for a focusable element. Control property now returns null.
While you are fixing this, if a software engineer reads this, can I offer more coding help? I recommend to install Resharper, your eyes will pop out your head with all the C# code warnings. Second, take a close look at KeyboardFocusHelper.cs. In GetElementToFocus(), on 1 line, there is "if (element is IInputElement)", and then right after the if, there is "if (element is FrameworkElement)". Well, the second if will never run because a FrameworkElement is a IInputElement. So what is the second if for? The same quirky 2 if statements can also be found in CanFocusResultCallback(). A UIElement is a IInputElement. So either the first if is redundant, or the second one. Also, testing for Focusable is not enough, you should include IsEnabled. See the method FocusHelper.CanBeFocused(), which does it right. That way, focus will go to an element that can actually receive focus. Although if nothing is enabled, the first focusable should be chosen.
(Note: Coupled with the fact that the PropertyGridControl is completely busted with a null exception immediately, which I see a ticket for, we'll be skipping 15.1.8.)