EnableExplicit

Define EventID

If OpenWindow(0,0,0,500,250,"Window",#PB_Window_SystemMenu|#PB_Window_ScreenCentered|#PB_Window_SizeGadget)
  ButtonGadget(1,10,10,150,20,"Fenstergröße fixieren",0)
  
Repeat
EventID=WaitWindowEvent()
If EventID=#PB_Event_Gadget
  Select EventGadget()
    Case 1
      SetWindowLongPtr_(WindowID(0),#GWL_STYLE,GetWindowLongPtr_(WindowID(0),#GWL_STYLE)&~#WS_THICKFRAME)
  EndSelect
EndIf
Until EventID = #PB_Event_CloseWindow
EndIf