D3D11 RHI flickers black border when resizing

Please check this video: flicker.mp4
Steps:
1) Set QT_WIDGETS_RHI=1 and QT_WIDGETS_RHI_BACKEND=d3d11
2) Run linguist.exe
3) Resize window quickly

This can be reproduced with any Qt application when using QT_WIDGETS_RHI=1 and QT_WIDGETS_RHI_BACKEND=d3d11.
Or adding any qml/webengine/openglwidget child to change the native window to be RHI widget.

Windows only.
OpenGL RHI is good.

Qt uses DXGI_SCALING_NONE mode. It seems that the black area comes from the swap chain's background color. (i.e. IDXGISwapChain1::SetBackgroundColor, default is 0,0,0,1)