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Best way to find out is uninstall Display Fusion. If problem goes away then you know (also if problem does not go away of course).
I know this thread is kinda old, but it took me a while to find a solution to this and I wanted to share it. I also have DisplayFusion. Open your DisplayFusion settings and go to the window management tab. In there, you'll see a section labelled "Alt+Tab Handler". Turn this off and the problem goes away. Hope this helps someone.