> In some frameworks, if you specify a dialog window with a parent of another window, they are
> min/max'd together and are not constrained as you described
That is also how Pivot windows behave. However, all Pivot windows live within the Pivot "display", which is itself entirely contained within a single native frame or applet (specifically, it lives in a "display host" which is a child of the native window). That's why Pivot windows appear to be constrained to the client area of the native window. This is by design - it gives Pivot complete control over the appearance of all UI elements including the window and its associated trim, and allows us to create a user experience that otherwise would not be possible (for example, "sheet" windows are supported on OS X but not on other platforms). Windows in other RIA frameworks behave similarly.
However, there may be times when opening multiple native frame windows in a Pivot application is desirable. Currently, this is possible but not easy. The solution proposed in this ticket would simplify the process by allowing an application to create multiple displays, each of which would be contained in its own display host and native frame.
That said, this is actually considered to be something of an edge case. Generally, when a Pivot application requires a dialog or an alert, the recommended practice is to use a sheet or a prompt instead. Sheets are attached to the top of the owner window and can't be moved by the user. As a result, they can't be dragged "off screen" like a frame can. Perhaps this approach would work for your application?