What's wrong with my understanding of this code?

;
#cs
    Script:        Desktop client size and Taskbar position (code snippet)
    Author:        Bruce Huber (cbruce)
    AutoIt:        v3.2.12.1
    Date:        2008.08.13 - Original release
 
    Purpose:
        Determine whether the Taskbar is visible or hidden and where it is docked on the Desktop.
        Also calculate the available Desktop client region, accounting for any Taskbar space.
#ce
 
;    Desktop and Taskbar position information - GetDesktopAndTaskbarPos().
Global Enum $DTP_X, $DTP_Y, $DTP_W, $DTP_H
Global $aDesktop_Pos, $aTaskbar_Pos
Global Enum $TBDL_UNKNOWN, $TBDL_LEFT, $TBDL_RIGHT, $TBDL_TOP, $TBDL_BOTTOM
Global $Taskbar_Dock_Location = $TBDL_UNKNOWN
Global $Taskbar_Visible
  
 
Func GetDesktopAndTaskbarPos()
    ;    Get the Desktop size - which includes the Taskbar space, if the Taskbar is visible.
    $aDesktop_Pos = WinGetPos( "Program Manager")
    ;    Get the Taskbar size.
    $aTaskbar_Pos = WinGetPos( "[Class:Shell_TrayWnd]")
    ;    Determine if Taskbar is taking up space on the Desktop.
    If ( $aTaskbar_Pos[$DTP_X] > -3) And ( $aTaskbar_Pos[$DTP_Y] > -3) And ( $aTaskbar_Pos[$DTP_X] < ( $aDesktop_Pos[$DTP_W] - 5)) And ( $aTaskbar_Pos[$DTP_Y] < ( $aDesktop_Pos[$DTP_H] - 5)) Then
        ;    Taskbar is Visible.
        $Taskbar_Visible = True
        ;    Calculate the parameters of the AVAILABLE Desktop client region.
        If ( $aTaskbar_Pos[$DTP_X] < 1) and ( $aTaskbar_Pos[$DTP_Y] > 0) and ( $aTaskbar_Pos[$DTP_W] > $aTaskbar_Pos[$DTP_H]) Then
            ;    Taskbar is on the BOTTOM.
            $Taskbar_Dock_Location = $TBDL_BOTTOM
            ;    We need to adjust the Desktop Height.
            $aDesktop_Pos[$DTP_H] = $aTaskbar_Pos[$DTP_Y] - 1
        ElseIf ( $aTaskbar_Pos[$DTP_X] < 1) and ( $aTaskbar_Pos[$DTP_Y] < 1) and ( $aTaskbar_Pos[$DTP_W] < $aTaskbar_Pos[$DTP_H]) Then
            ;    Taskbar is on the LEFT.
            $Taskbar_Dock_Location = $TBDL_LEFT
            ;    We need to adjust the Desktop X and Width.
            $aDesktop_Pos[$DTP_X] = $aTaskbar_Pos[$DTP_X] + $aTaskbar_Pos[$DTP_W] + 1
            $aDesktop_Pos[$DTP_W] = $aTaskbar_Pos[$DTP_X] - 1
        ElseIf ( $aTaskbar_Pos[$DTP_X] > 0) and ( $aTaskbar_Pos[$DTP_Y] < 1) and ( $aTaskbar_Pos[$DTP_W] < $aTaskbar_Pos[$DTP_H]) Then
            ;    Taskbar is on the RIGHT.
            $Taskbar_Dock_Location = $TBDL_RIGHT
            ;    We need to adjust the Desktop Width.
            $aDesktop_Pos[$DTP_W] = $aTaskbar_Pos[$DTP_X] - 1
        ElseIf ( $aTaskbar_Pos[$DTP_X] < 1) and ( $aTaskbar_Pos[$DTP_Y] < 1) and ( $aTaskbar_Pos[$DTP_W] > $aTaskbar_Pos[$DTP_H]) Then
            ;    Taskbar is on the TOP.
            $Taskbar_Dock_Location = $TBDL_TOP
            ;    We need to adjust the Desktop Y and Height.
            $aDesktop_Pos[$DTP_Y] = $aTaskbar_Pos[$DTP_Y] + $aTaskbar_Pos[$DTP_H] + 1
            $aDesktop_Pos[$DTP_H] = $aTaskbar_Pos[$DTP_Y] - 1
        Else
            ;    Where the heck has the Taskbar gone?
            $Taskbar_Dock_Location = $TBDL_UNKNOWN
        EndIf
    Else
        ;    Taskbar is Hidden.
        $Taskbar_Visible = False
    EndIf
    #cs
            $aDesktop_Pos now contains parameters that only define the AVAILABLE Desktop client region.  This will
        be the entire Desktop if the Taskbar is hidden.  Otherwise, the region will be the Desktop area minus the
        Taskbar area.
 
            $Taskbar_Dock_Location now specifies WHERE, on the Desktop, that the Taskbar is docked.
    #ce
EndFunc   ; GetDesktopAndTaskbarPos()
;

Global Const $MONITOR_DEFAULTTONULL     = 0x00000000
Global Const $MONITOR_DEFAULTTOPRIMARY  = 0x00000001
Global Const $MONITOR_DEFAULTTONEAREST  = 0x00000002
 
Global Const $CCHDEVICENAME             = 32
Global Const $MONITORINFOF_PRIMARY      = 0x00000001
 
 
$hMonitor = GetMonitorFromPoint(0, 0)
;$hMonitor = GetMonitorFromPoint(-2, 0)
;$hMonitor = GetMonitorFromPoint(@DesktopWidth, 0)
 
If $hMonitor <> 0 Then
    Dim $arMonitorInfos[4]
    If GetMonitorInfos($hMonitor, $arMonitorInfos) Then _
        Msgbox(0, "Monitor-Infos", "Rect-Monitor" & @Tab & ": " & $arMonitorInfos[0] & @LF & _
                            "Rect-Workarea" & @Tab & ": " & $arMonitorInfos[1] & @LF & _
                            "PrimaryMonitor?" & @Tab & ": " & $arMonitorInfos[2] & @LF & _
                            "Devicename" & @Tab & ": " & $arMonitorInfos[3])
EndIf
 
Exit
 
Func GetMonitorFromPoint($x, $y)
    $hMonitor = DllCall("user32.dll", "hwnd", "MonitorFromPoint", _
                                            "int", $x, _
                                            "int", $y, _
                                            "int", $MONITOR_DEFAULTTONULL)
    Return $hMonitor[0]
EndFunc
 
 
Func GetMonitorInfos($hMonitor, ByRef $arMonitorInfos)
    Local $stMONITORINFOEX = DllStructCreate("dword;int[4];int[4];dword;char[" & $CCHDEVICENAME & "]")
    DllStructSetData($stMONITORINFOEX, 1, DllStructGetSize($stMONITORINFOEX))
 
    $nResult = DllCall("user32.dll", "int", "GetMonitorInfo", _
                                            "hwnd", $hMonitor, _
                                            "ptr", DllStructGetPtr($stMONITORINFOEX))
    If $nResult[0] = 1 Then
        $arMonitorInfos[0] = DllStructGetData($stMONITORINFOEX, 2, 1) & ";" & _
            DllStructGetData($stMONITORINFOEX, 2, 2) & ";" & _
            DllStructGetData($stMONITORINFOEX, 2, 3) & ";" & _
            DllStructGetData($stMONITORINFOEX, 2, 4)
        $arMonitorInfos[1] = DllStructGetData($stMONITORINFOEX, 3, 1) & ";" & _
            DllStructGetData($stMONITORINFOEX, 3, 2) & ";" & _
            DllStructGetData($stMONITORINFOEX, 3, 3) & ";" & _
            DllStructGetData($stMONITORINFOEX, 3, 4)
        $arMonitorInfos[2] = DllStructGetData($stMONITORINFOEX, 4)
        $arMonitorInfos[3] = DllStructGetData($stMONITORINFOEX, 5)
    EndIf
 
    Return $nResult[0]
EndFunc