Clock panel applet fades when not focussed
Possibly other things too, but I've only noticed it with Clock as I don't use the panel for that much. I'd expect it to be readable at a glance whichever window is actually focussed, not faded.
Probably related to GTK+3 theme and how things react to _NET_WM_STATE_FOCUSED. xfce4-datetime-plugin did not do this.
Attached image shows Clock (left) and Datetime (right) when panel does not have focus.