NavigationView should have a way to set CornerRadius and BorderThickness of the content in left-pane mode

In left-pane expanded or collapsed modes, the Content of the NavigationView is shown with a rounded upper-left corner while the other corners are not rounded. Not everyone wants this look! It should be possible to set the CornerRadius of the NavigationView's content (or at least have it equal to the CornerRadius of the NavigationView itself). There also seems to be a very thin 1 dpi border around the content (in left modes, and above the content in top modes), which is not always desirable. I looked through the control templates but could not find the source of either one. These should be definable by the user, either through resources or properties.

Start by locating the NavigationView control templates mentioned in the issue and inspect how the content's corner and border visuals are defined in left- and top-pane modes. Determine whether resource or property customization is appropriate; done means users can control the content CornerRadius and BorderThickness without changing unrelated NavigationView visuals.