shrink a window down (while honoring MinWidth and minHeight) so that it fits on
whatever monitor it’s on. 
First we have a function to determine which monitor the window’s on:

Protected Function ScreenNumber() As Integer
  // Return the number of the screen (from 0 to ScreenCount-1)
  // which this window is mostly on.
  Dim bestScreen As Integer = -1
  Dim bestArea As Integer
  for i As Integer = 0 to ScreenCount-1
    Dim w As Integer = _
       Min( Screen(i).Left + Screen(i).Width, Left + Width ) _
    – Max( Screen(i).Left, Left )
    Dim h As Integer = _
       Max( Screen(i).Top + Screen(i).Height, Top + Height ) _
    – Max( Screen(i).Top, Top )
    if w < 0 or h bestArea then
      bestScreen = i
      bestArea = area
    end if
  next
  if bestScreen sBottom then
    Height = Max( MinHeight, sBottom – Top )
  end if
 
  if Left + Width > sRight then
    Width = Max( MinWidth, sRight – Left )
  end if 
End Sub

You could throw both of these methods into a Window, add a button that
calls FitToScreen, and then drag your window around to various funky
positions on both monitors and make sure it does what it should.