If you have upgraded your OS X Lion system to version 10.7.4, you may run into an issue where the log-in screen might appear black and not show the standard usernames and icons and the area to present your password. When this issue happens, if you move your mouse the system will reveal a small square of the log-in window underneath the mouse, and subsequent movement of this square will erase more of the black mask and further reveal the log-in window.
While perhaps confusing and odd for the user, this issue does not break the functionality of the OS X log-in window, and once users have revealed their account pictures, they can click on them and then provide their password as usual to log in.
The issue in this case is a bug in how the system updates the screen output after the log-in window is revealed, and Apple has identified it as a problem with what is being drawn on the screen when the system is automatically logged out. Apparently on some systems if the display is in sleep mode when the account is logged out, the black output to the screen will remain persistent when the system is woken back up, and the system without a full-screen refresh the screen's elements will only show through this mask if they are individually refreshed by movement or focus.
This bug began appearing for some users after the latest OS X 10.7.4 update, and while a true fix will have to come with an OS update from Apple (likely with the upcoming OS X 10.7.5 update), there are a few workarounds that you can implement to prevent this bug from occurring:
- Adjust display sleep timings
Since this problem happens when the display is sleeping before the system automatically logs out, an easy workaround is to change the timings either for the display sleep or when the system automatically logs out to be so the display will not sleep before the system logs out. To adjust the automatic log-out timings, go to the "General" section of the Security & Privacy system preferences, and click Advanced button. In the settings pane that shows up, adjust the time to be less than that for the display sleep, which can be found in the Energy Saver system preferences and which can optionally be adjusted to be greater than the automatic log-out time.
- Disable automatic log-out
This issue only affects systems that have the automatic log-out feature enabled, so one option if you do not need this feature is to disable it entirely. In the same area where you would change the automatic log-out times, one option is to uncheck this feature to disable it altogether.