[VC/MFC]why the app window is affected by other items which move across it?

I've made two screen shots in order t开发者_开发技巧o make the problem specified.

The normal style:

the affected style after I move some windows such as browsers across it:

I wonder why this happened?And How can fix this problem?I'm working with VS2008 on Windows.

Thanks in advance!

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When this has happened to me - it's due to not handling the OnPaint calls correctly. That's the area I would look at