There is a problem when fast-switching (Cmd-Tab shortcut) between full-screen applications.
In the demonstration video below, you can see that I enter full-screen in iTunes. Then I fast-switch to Safari. Then I fast-switch back to iTunes. What we get is iTunes with whatever other window happens to be open. (In the video the Equalizer window is open). If the Equalizer window was not open, then we would see NO windows at all for iTunes, which shows how ridiculous this bug is.) In order to see the window that was full-screened, that window needs to be manually selected in the Window menu item/the application icon in the Dock must be clicked.
In the video, here are the steps:
- Make the application full-screen (iTunes).
- Fast-switch (cmd-tab shortcut) to another application (Safari).
- Fast-switch back to iTunes.
- The window that was full-screen now doesn’t show, only another non-full-screen window (in this case the Equalizer).
- In order to get to the full-screen window, the menu item Window > (window name) must be selected, or the application Dock icon must be clicked.
OS X El Capitan 10.11.6