I had that same question.
Achieve solve follows.
First you must calculate the difference between the rectangle of the form and the slidepanel like you have to consider the original location x,y of the object.
After this in the resize event of the form add the following
If Not SlidePanel1.IsOpen Then
SlidePanel1.OpenBounds = New System.Drawing.Rectangle(OriginalX, OriginalY, Me.DisplayRectangle.Width - DifferenceWidth, Me.DisplayRectangle.Height - DifferenceHeight)
are variables and constants which you must declare or set manually.
Sorry, English is not my native language using a translator to answer
Greetings from Venezuela