Public Class UserControl1
    Private Sub UserControl1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        AddHandler Me.Parent.Resize, AddressOf ParentResize
    End Sub

    Sub ParentResize(ByVal sender As Object, ByVal e As System.EventArgs)

    End Sub
End Class

Dim WithEvents MyParent as Form

Sub Init ()
    MyParent = Me.Container
End Sub

Как-то так, у меня нет VB6 поэтому я мог где-то ошибиться в синтаксисе или названии процедур, но смысл такой - объявляешь некую переменную с ключевым словом WithEvents и сможешь доступаться к ее событиям. Возможно заработает.

Точнее, как поймать событие resize парент-окна из кода ю-котрола?

Кстатти, в тему, а можно осуществить на .Нет такое:
нарисовать на контроле методами gdi+ что-то, сделать сам контрол прозрачным. так, чтобы нарисованное было видно(это я знаю, что моно) и
оно обрабатывало mousedown/move ?

А можно сделать регион по гарисованной форме (незамкнутой)?

В 6-ке тебе нужно юзать сабклассинг - успехов!

'      ___            ___
'     /\__\          /\__\
'    /:/ _/_        /:/  /
'   /:/ /\  \      /:/  /___
'  /:/ /::\  \    /:/  //\  \
' /:/_/:/\:\__\  /:/__/ \:\__\
' \:\/:/ /:/  /  \:\  \ /:/  /
'  \::/ /:/  /    \:\  /:/  /
'   \/_/:/  /      \:\/:/  /
'     /:/  /        \::/  /
'     \/__/          \/__/
'   STARGAZER        GROUP

Option Explicit
Option Base 1

'Begin Definitions
'   Begin API
        Private Declare Function CallWindowProcA Lib "user32" (ByVal lpPrevWndFunc As Long, ByVal lngHandle As Long, ByVal lngMsg As Long, ByVal lngFirstParam As Long, ByVal lngLastParam As Long) As Long
        Private Declare Function SetWindowLongA Lib "user32" (ByVal lngHandle As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
        Private Declare Function GetWindowRect Lib "user32" (ByVal hWnd As Long, lpRect As RectEx) As Long
        Private Declare Function GetCursorPos Lib "user32" (pntPoint As Point) As Long
        Private Declare Function SystemParametersInfoA Lib "user32" (ByVal uAction As Long, ByVal uParam As Long, ByRef lpvParam As Any, ByVal fuWinIni As Long) As Long
        Private Declare Sub RtlMoveMemory Lib "kernel32" (Destination As Any, Source As Any, ByVal Length As Long)
        Private Declare Function EnumWindows Lib "user32" (ByVal lpEnumFunc As Long, ByVal lParam As Long) As Long
        Private Declare Function IsWindowVisible& Lib "user32" (ByVal hWnd As Long)
        Private Declare Function GetParent& Lib "user32" (ByVal hWnd As Long)
        Private Declare Function GetDesktopWindow Lib "user32" () As Long
        'debug
        Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hWnd As Long, ByVal lpString As String, ByVal cch As Long) As Long
        Private Declare Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLengthA" (ByVal hWnd As Long) As Long
        'Private Declare Function GetClassLong Lib "user32" Alias "GetClassLongA" (ByVal hwnd As Long, ByVal nIndex As Long) As Long
'   End API
'   Begin Types
        'API need it
        Private Type Point
            X As Long
            Y As Long
        End Type
        
        'rectangle of related window
        Private Type RectEx
            Left     As Long
            Top      As Long
            Right    As Long
            Bottom   As Long
            'following enhancement can do some bad things, but this makes code easier
            Width    As Long
            Height   As Long
            OldWProc As Long
            hWnd     As Long
        End Type
'   End Types
'   Begin Variables
        
        Public RelWnd() As RectEx, RWCount As Long    'Related Windows
        Private m_Borders() As RectEx, BCount As Long 'All lines to join
        Private m_Enabled As Boolean, m_Size As Long  'Propertie's inner variables
        Private SPoint As Point                       'Coordinates of cursor when move started
        
        'this should be inside WndProc, but it cause memory loses
        Private WScreen As RectEx 'Properties of screen(without taskbar)
        Private CPoint As Point   'Current Point
        Private Iter As Long      'Cycle iterator
        Private CurrRW As Long    'Window which is moving now
        Private TRect As RectEx   'temporary rectangle
        Private TMP0 As Long
'   End Variables
'End
'Begin Code
    Private Sub StartHook() 'this procedures starts subclassing for all related windows
        Dim I As Long
        For I = 1 To RWCount
            With RelWnd(I)
                If .hWnd <> 0 Then
                    .OldWProc = SetWindowLongA(.hWnd, -4, AddressOf WndProc) 'I don't like this.
                End If
            End With
        Next I
    End Sub
    
    Private Sub StopHook() '...and VERSUS VERSA
        Dim I As Long
        For I = 1 To RWCount
            With RelWnd(I)
                If .hWnd <> 0 Then
                    Call SetWindowLongA(.hWnd, -4, .OldWProc) 'I don't like this too.
                End If
            End With
        Next I
    End Sub
    
    Private Function WndProc(ByVal Handle As Long, ByVal Msg As Long, _
                             ByVal FParam As Long, ByVal lParam As Long) As Long
        
        'eats 10% of processor time on Celeron 2GHz. VB !
        'WARNING: never use STOP,END,Breakpoints here, be careful with errors here.
        '         This causes application hangs and fatal errors!
        
        'Get RelWnd item number with this handle
        For Iter = 1 To RWCount
            If RelWnd(Iter).hWnd = Handle Then CurrRW = Iter: Exit For
        Next Iter
        
        If Msg = &HA1 Then 'start dragging
            
            If FParam = 2 Then
                
                SystemParametersInfoA 48, 0, WScreen, 0   'gets an actual screen size to glue to taskbar, but not to bottom of the screen
                GetCursorPos CPoint                       'get the position of cursor in start of form dragging
                GetWindowRect Handle, RelWnd(CurrRW)      'what does user drag?
                
                SPoint.X = CPoint.X - RelWnd(CurrRW).Left 'fill variables for moving
                SPoint.Y = CPoint.Y - RelWnd(CurrRW).Top
                
            End If
            
        ElseIf Msg = &H216 Then 'this is unknown sysevent. Danila, what's this?
            
            GetCursorPos CPoint                           'coordinates of cursor to get the DX and DY of moving
            GetWindowRect Handle, RelWnd(CurrRW)
            
            With RelWnd(CurrRW)
                TMP0 = CPoint.X - .Left - SPoint.X        'This helps not to count this twice.
                TRect.Left = .Left + TMP0                 'Just a good optimization
                TRect.Right = .Right + TMP0
                TMP0 = CPoint.Y - .Top - SPoint.Y
                TRect.Top = .Top + TMP0
                TRect.Bottom = .Bottom + TMP0
                TRect.Width = .Right - .Left
                TRect.Height = .Bottom - .Top
            End With
            
            BCount = 0
            ReDim m_Borders(1)
            EnumWindows AddressOf EnumWinProc, 0
            
            With TRect
                If Near(.Right, WScreen.Right, m_Size) Then
                    .Left = WScreen.Right - .Width
                    .Right = WScreen.Right
                ElseIf Near(.Left, WScreen.Left, m_Size) Then
                    .Left = WScreen.Left
                    .Right = WScreen.Left + .Width
                End If
                If Near(.Bottom, WScreen.Bottom, m_Size) Then
                    .Top = WScreen.Bottom - .Height
                    .Bottom = WScreen.Bottom
                ElseIf Near(.Top, WScreen.Top, m_Size) Then
                    .Top = WScreen.Top
                    .Bottom = WScreen.Top + .Height
                End If
            End With
            
            For Iter = 1 To BCount
                With m_Borders(BCount)
                    Debug.Print "0"
                    If Cross(TRect.Top, TRect.Bottom, .Top, .Bottom) Then
                        Debug.Print "1"
                        If Near(TRect.Left, .Right, m_Size) Then
                            Debug.Print "11"
                            TRect.Left = .Right
                            TRect.Right = .Right + TRect.Width
                        ElseIf Near(TRect.Right, .Left, m_Size) Then
                            Debug.Print "12"
                            TRect.Left = .Left - TRect.Width
                            TRect.Right = .Left
                        End If
                    End If
                    If Cross(TRect.Left, TRect.Right, .Left, .Right) Then 'this properties are already changed
                        Debug.Print "2"
                        If Near(TRect.Top, .Bottom, m_Size) Then
                            Debug.Print "21"
                            TRect.Top = .Bottom
                            TRect.Bottom = .Bottom + TRect.Height
                        ElseIf Near(TRect.Bottom, .Top, m_Size) Then
                            Debug.Print "22"
                            TRect.Top = .Bottom - TRect.Width
                            TRect.Bottom = .Top
                        End If
                    End If
                End With
            Next Iter
            
            With TRect
                'in VB you can't say: RelWnd(CurrRW) = TRect like in VB .NET
                RelWnd(CurrRW).Left = TRect.Left
                RelWnd(CurrRW).Top = TRect.Top
                RelWnd(CurrRW).Bottom = TRect.Bottom
                RelWnd(CurrRW).Right = TRect.Right
            End With
            
            RtlMoveMemory ByVal lParam, RelWnd(CurrRW), 16 'Len(rect)
            
        End If
        
        WndProc = CallWindowProcA(RelWnd(CurrRW).OldWProc, Handle, Msg, FParam, lParam)
        
    End Function
    
    Private Function EnumWinProc(ByVal hWnd As Long, ByVal lParam As Long) As Long
        Dim I As Long, txt As String
        'WARNING: never use STOP,END,Breakpoints here, be careful with errors here.
        '         This causes application hangs and fatal errors!
        If IsWindowVisible(hWnd) And _
            GetParent(hWnd) = 0 And _
            hWnd <> GetDesktopWindow Then 'this is very strong laws. Some windows could be lost
            For I = 1 To RWCount
                If RelWnd(I).hWnd = hWnd Then I = -1: Exit For
            Next I
            txt = Space$(256) '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
            GetWindowText hWnd, txt, 256
            txt = Left(txt, GetWindowTextLength(hWnd))
            If txt = "Program Manager" Then I = -1
            If I <> -1 Then
                BCount = BCount + 1
                ReDim Preserve m_Borders(BCount)
                GetWindowRect hWnd, m_Borders(BCount)
                'With m_Borders(BCount)
                '    .Width = .Right - .Left
                '    .Height = .Bottom - .Top
                'End With
            End If
        End If
        EnumWinProc = 1
    End Function
    
    'Private Function InLimit(TheSmall As Long, Num As Long, TheBig As Long) As Boolean
    '    InLimit = (TheSmall <= Num) And (Num <= TheBig)
    'End Function
    
    Private Function Near(Num As Long, NearTo As Long, Accur As Long) As Boolean
        Near = (NearTo - Accur <= Num) And (Num <= NearTo + Accur)
    End Function
    
    Private Function Cross(s1 As Long, e1 As Long, s2 As Long, e2 As Long) As Boolean
        'cross intervals
        Cross = (e1 >= s2) Or (e2 >= s1)
    End Function
'End Code
'Begin Properties
    Public Property Get Size() As Long
        Size = m_Size
    End Property
    Public Property Let Size(ByVal New_Size As Long)
        m_Size = New_Size
    End Property
    
    Public Property Get Enabled() As Boolean
        Enabled = m_Enabled
    End Property
    Public Property Let Enabled(ByVal New_Enab As Boolean)
        If m_Enabled <> New_Enab Then
            If New_Enab Then StartHook Else StopHook
        End If
        m_Enabled = New_Enab
    End Property
'End Properties

Хреновенький пример: