You could make a popup class that uses the adorner layer to put itself ontop of everything else.
Make a base class for your popup that has a property called IsOpen and when it is changed set the controls visibility to the appropriate value.
To stop the controls that are underneath your popup being clicked you can make the popup take of the full size of the page. You would have it mostly transparent except for the actuall middle where the popup. If you wanted it to be completly transparent execpt for the popup you would need to override the HitTestCore on your popup.
Something like this:
protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
{
// We want this control to behaive as a single rectangle and we don't much care
// whether or it has a solid background. So we override this method so we can have // mouse over info for the entire panel regardless of background.
// run the base hit test first, because if it finds something we don't want to overrule it.
HitTestResult result = base.HitTestCore(hitTestParameters);
// If we didn't get a hit generate a new hit test result, because HitTestCore is never called unless
// the mouse is over the controls bounding rectangle.
if (result == null)
result = new PointHitTestResult(this, hitTestParameters.HitPoint);
return result;
}
I hope this can point you in the right direction.