I have a Parent form with multiple child forms appearing in it.
On all the child forms I have turned off all the Min/Max button options an set the border to ‘none’.
Yet there is a small border appearing in between the main form window border and the Menustrip control docked to the top of the main form.
What is this additional layer, and how do I get rid of it.

The answer was as per @Hans Passants comment
It is a bug in Winforms, it forgets to throw an exception when you try to create an MDI child form without a border. MDI children must have a border. If you don’t want to make the child windows resizable then you don’t have a use for MDI.
Thank you