@Simon Mourier, Thanks for your thoughts. We confirmed that it gets back to iFlip mode after the ovelapping windows disappear. But we just don't know how many frames it takes to get back to iFlip mode. I was wondering if there is a solution to request/force/fool the DWM to re-evalulate the current state immediately and switch to iFlip mode sooner than it usually takes.

something like the following steps:

swapChain->SetFullscreenState(TRUE, nullptr);
swapChain->ResizeBuffers(...);
swapChain->Present(0, 0);

But I could not find any authentic source that gives a guranteed and safer solution.