If we click on the full screen button of the flash in the secondary monitor then if you really look at the x, y cordinate of that full screen window, it will be out of range of the secondary monitor resolution.
So just after making this window as a child of some other native/qt window just move your full screen window to 0, 0 position.
I achieved in windows using MoveWindow(hwnd, x, y , width , height), and then it is visible.