关于动态拉动窗口分割线

初学VB，请不吝赐教！
在主窗体中，有三个控件，左为richtextbox1,右为text1，中间image1为作为分割条。
期望在左右拖动image1时，能实时动态地改变richtextbox1和text1的区域。
运行后，有2个问题。一是在按住鼠标拖动image1时，image1在不停地闪烁；二是3个控件都没有到鼠标指针到达的地方。请问各位高手，我这种设计思路合适吗？如合适，问题出在哪里？
Dim mdown As Boolean
Dim p1, p2, p3 As Single
Private Sub Image1_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
  p1 = Image1.Left
  p2 = Text1.Left
  p3 = RichTextBox1.Width
  Image1.Visible = True
  mdown = True  
End Sub

Private Sub Image1_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
If mdown = True Then
  Text1.Left = p2 + X
  Image1.Left = p1 + X
  RichTextBox1.Width = p3 + X
End If
End Sub

Private Sub Image1_MouseUp(Button As Integer, Shift As Integer, X As Single, Y As Single)
    mdown = False
    Image1.Visible = False
End Sub