Finalmente acertamos!
x := (A_ScreenWidth/2)-(Width/2)
y := (A_ScreenHeight/2)-(Height/2)
Gui, Show, %x% %y% w200 h50
Gui, Add, Text, vCount w150, PgDn pressed 0 times.
Count := 0
$PgDn:: ;Default behavior is to block keystroke, ~ allows it to pass through
Count := Count + 1
GuiControl,,Count, PgDn pressed %Count% times.
KeyWait, PgDn ;Wait for PgDn to be released
return
$PgUp:: ;Default behavior is to block keystroke, ~ allows it to pass through
Count := Count - 1
GuiControl,,Count, PgUp pressed %Count% times.
KeyWait, PgUp ;Wait for PgUp to be released
return
GuiClose:
ExitApp
return