If I invoke Exposé and use the gesture to swipe between fullscreen apps, open windows from other applications will show (similar to ⌘+⇥). However, I cannot click on them to bring them to the front, so it is practically useless.
I noticed this behavior since Sierra and wonder if this is intended behavior or just a bug.
Bonus question (Since I don't have access to macOS 10.11 or lower): Has this behavior existed before?
Best Answer
My interpretation of the question's scenario is:
If you reproduce this state and then perform a downward three- or four-fingered swipe, the windows will "bob" a little and the blue border highlight will reappear; clicking on the highlighted window will bring it to focus and exit Exposé, which I suppose is the expected behavior. (I believe this is what an earlier answer was alluding to.) The arrow keys will also cause the blue highlight to reappear and allow a window to be selected.
My personal opinion is that the initial lack of response to mouseovers is a bug and not intended, but I have no evidence either way. Hopefully these workarounds help, though.