I read the code in qvulkanwindow.cpp and find swapchian will be recreated when

if (q->size() * q->devicePixelRatio() != swapChainImageSize) {        
recreateSwapChain();
}

in beginFrame(). However, in recreateSwapChain(), I find the swapChainImageSize will actually be changed to

swapChainImageSize = QSize(bufferSize.width, bufferSize.height); 

and bufferSize comes from

VkExtent2D bufferSize = surfaceCaps.currentExtent; 

So in some case, if bufferSize didn't match swapChainImageSize, it will be recreated again and again.

In my case, this happened when my device devicePixelRatio()=1.5 and I use mouse change the window size. And the difference betwenn bufferSize and q->size() * q->devicePixelRatio() is actually just 1. 

But when devicePixelRatio()=1 it didn't happen. I wonder is it caused by float problem?