Ограничение зоны перетаскивания форм/окон

Option Explicit
 
Private Type WINDOWPOS
    hwnd            As Long
    hWndInsertAfter As Long
    x               As Long
    y               As Long
    cx              As Long
    cy              As Long
    flags           As Long
End Type
 
Private Declare Function FindWindowEx Lib "user32" _
                         Alias "FindWindowExW" ( _
                         ByVal hWndParent As Long, _
                         ByVal hWndChildAfter As Long, _
                         ByVal lpClassName As Long, _
                         ByVal lpWindowName As Long) As Long
Private Declare Sub memcpy Lib "kernel32" _
                    Alias "RtlMoveMemory" ( _
                    ByRef Destination As Any, _
                    ByRef Source As Any, _
                    ByVal Length As Long)
                    
Private Const WM_MDIGETACTIVE         As Long = &H229
Private Const WM_WINDOWPOSCHANGING    As Long = &H46
Private Const SWP_NOMOVE              As Long = &H2
 
Private WithEvents m_cHookChild As CTrickSubclass
Private WithEvents m_cHook      As CTrickSubclass
 
Private m_hCurWnd   As OLE_HANDLE
 
Private Sub m_cHook_WndProc( _
            ByVal hwnd As OLE_HANDLE, _
            ByVal lMsg As Long, _
            ByVal wParam As Long, _
            ByVal lParam As Long, _
            ByRef lRet As Long, _
            ByRef bDefCall As Boolean)
 
    Select Case lMsg
    Case WM_MDIGETACTIVE
        
        hwnd = m_cHook.CallDef(hwnd, lMsg, wParam, lParam, False)
        
        If hwnd <> m_hCurWnd Then
            m_cHookChild.Hook hwnd
            m_hCurWnd = hwnd
        End If
        
        lRet = hwnd
        bDefCall = False
        
    End Select
 
End Sub
 
Private Sub m_cHookChild_WndProc( _
            ByVal hwnd As OLE_HANDLE, _
            ByVal lMsg As Long, _
            ByVal wParam As Long, _
            ByVal lParam As Long, _
            ByRef lRet As Long, _
            ByRef bDefCall As Boolean)
    Dim tWndPos As WINDOWPOS
    
    Select Case lMsg
    Case WM_WINDOWPOSCHANGING
        
        memcpy tWndPos, ByVal lParam, Len(tWndPos)
        
        If (tWndPos.flags And SWP_NOMOVE) = 0 Then
            
            If tWndPos.x < 0 Then
                tWndPos.x = 0
                memcpy ByVal lParam, tWndPos, Len(tWndPos)
            End If
            
            bDefCall = False
        
        End If
        
    End Select
    
End Sub
 
Private Sub MDIForm_Load()
    Dim frm As Form
    Dim i As Long
    
    Set m_cHook = New CTrickSubclass
    Set m_cHookChild = New CTrickSubclass
    
    m_cHook.Hook FindWindowEx(Me.hwnd, 0, StrPtr("MDICLIENT"), 0)
    
    For i = 0 To 15
    
        If Rnd > 0.5 Then
            Set frm = New Form1
        Else
            Set frm = New Form2
        End If
        
        frm.Visible = True
        
    Next
    
End Sub
 
Private Sub MDIForm_QueryUnload(Cancel As Integer, UnloadMode As Integer)
    m_cHook.Unhook
    m_cHookChild.Unhook
End Sub

Private Sub m_cHookChild_WndProc( _
            ByVal hWnd As OLE_HANDLE, _
            ByVal lMsg As Long, _
            ByVal wParam As Long, _
            ByVal lParam As Long, _
            ByRef lRet As Long, _
            ByRef bDefCall As Boolean)
    Dim tWndPos As WINDOWPOS
    
    Select Case lMsg
    Case WM_WINDOWPOSCHANGING
        
        memcpy tWndPos, ByVal lParam, Len(tWndPos)
                  
        If tWndPos.flags = &H214 Then
            If tWndPos.x < 0 Then
                tWndPos.x = 0
                memcpy ByVal lParam, tWndPos, Len(tWndPos)
            End If
            
            bDefCall = False
        End If
        If tWndPos.flags = &H1 Then
            If tWndPos.x < 0 Then
                tWndPos.x = 0
                memcpy ByVal lParam, tWndPos, Len(tWndPos)
            End If
            
            bDefCall = False
        End If
        If tWndPos.flags = &H1000214 Then
            If tWndPos.x < 0 Then
                tWndPos.x = 0
                memcpy ByVal lParam, tWndPos, Len(tWndPos)
            End If
            
            bDefCall = False
        End If
        If tWndPos.flags = &H205 Then
            If tWndPos.x < 0 Then
                tWndPos.x = 0
                memcpy ByVal lParam, tWndPos, Len(tWndPos)
            End If
            
            bDefCall = False
        End If
    End Select
End Sub