I'm not sure who all knew this, but if you want to create a dialog window in Blitz+ all you have to do is create your main window then create the dialog window and make it a child gadget to the main window.
Example:
mainwindow=CreateWindow("TEST",50,50,640,480,Desktop(),1+2)
button=CreateButton("BABY!",50,50,150,150,mainwindow)
dialogwindow=0
Repeat
Select WaitEvent()
Case $803
Select EventSource()
Case mainwindow
End
Case dialogwindow
FreeGadget dialogwindow
dialogwindow=0
EnableGadget mainwindow
ActivateGadget mainwindow
End Select
Case $401
Select EventSource()
Case button
If dialogwindow=0
DisableGadget mainwindow
dialogwindow=CreateWindow("DIALOG",1024/2-250/2,768/2-250/2,250,250,mainwindow,1)
EndIf
End Select
End Select
Forever
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