Child foreground niceness sometimes not applied

[jacobgkau]

Steps to reproduce:

• Fresh boot/login
• Open a terminal, run top, open Firefox and Nautilus
• Observe in top that the foreground/background nicenesses work when switching between Firefox and Nautilus
• Switch focus to the first terminal, then open a second terminal
• Run stress -c 1
• Observe that stress (and sometimes top) have the background niceness of 6, even though the terminal with stress is focused.
• Switch focus away from the terminal and back; the niceness of stress switches to 0.

This may occur more often if stress is run very quickly after switching focus to the terminal(s). However, it's not just a delay before the foreground niceness is applied; it's never applied until another focus switch when this issue occurs.

This is not caused by or fixed by #92.