Why did the background color of the form change?

Question

C++ / C#

When I changed the value of the IsMdiContainer property of the form to true the background color changed to gray instead of white. Why? When I set the value back to false, the background is also returned. How can I set True and return white color? Maybe I'm blind and didn't see the background color property, or I screwed something up with the code, or maybe it's my cat sitting under the table and changing the value???

a minute later:
OH MY GOD! After launching the application with a gray background, it turned into THIS!

Answer 1

MdiContainer is a container for forms (a form inside which other forms are placed if simplified) - you turn it on with the IsMdiContainer option. it has a different design and properties, so it looks different.
it is used differently than the usual forms of
referencing