PE>
  > ie I basically want the dialog to take up all the client
  > space of the first window, and if I move the main window,
  > the dialog moves with it.
PE>

  Make the parent of the dialogue the main window (you are currently using
  HWND_DESKTOP as the parent).  Whenever the main window's client window
  procedure receives a WM_SIZE, resize the dialogue using WinSetWindowPos
  so that it occupies the entire client window.

  > JdeBP <
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