C++ direct3D11 Why child window overflows parent

I have a direct3d11 buffer in a window which is child of another window. When I resize window from left side too fast that content of directx which is child of parent overflows. How can I prevent it ?

VIDEO: https://www.youtube.com/watch?v=mIxXT0zkX-k
there is an on-size event, when it is happening, you can stop drawing for a few MS or something.
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