I'm using WPF 4's (in .NET 4) ribbon control, version 3.5.40729.1, in my application (edit: It also happens with the newly released version 184.108.40.20619). The application menu and all sub-menu popups align as far left as possible, usually far outside the
ribbon window. The expected location is that the application menu's left edge aligns with the ribbon window's left edge, and that sub-menu popups will align with the left edge of the right (usually recent items) content pane.
Even if just create a brand new ribbonwindow project and run it, the bug happens.
I've attached a screenshot to illustrate.
I also posted about this issue on the msdn forums: