I've been using this software for a while now, and have made some posts about this issue with a particular game. I no longer believe it to be restricted to this game, as it's been happening without it running occasionally. It isn't something easy to reproduce, but all of a sudden I will press 'alt+tab' and the normal window shows up, and I can even tab through the different windows that are open. When I let go of both buttons however, the program doesn't switch over. I can click the program in the taskbar (on another monitor if whatever I'm running is in fullscreen), but it's a bit of a pain. Even restarting DF doesn't fix this issue.

What does fix the issue?
I went to task manager and ended 'DisplayFusion.exe', and started the program again. Same issue. Then I ended the 'DisplayFusionHookAppWIN6032.exe', nothing happened. Then, I ended 'DisplayFusionHookAppWIN6064.exe', and everything was back to normal. Those last two AppHook processes didn't close when I exited DisplayFusion the first time, but clearly something was bugged with the 64-bit process. Is there an advanced option that might assist with preventing this issue? I can start logging, however I don't know if I can reproduce it for a few days...maybe I'll wait until it happens and start logging. Any preferred level (1-3) of logging so you get just enough info, but not too much?

I've had this issue happen to me only twice since your post. Having two monitors running, one with a game in borderless/windowed fullscreen mode, and Chrome on the same screen as well. After opening DisplayFusion from the notification icon and setting the logging level to 1, I was immediately able to Alt+Tab again like usual.

I'm gonna leave my log on L1 for the time being to try to get this bug caught...