I am having a problem with xfce4-panel not positioned on the top row of the screen at startup. The result is that maximized windows over(under)lap the panel in for a few pixels. This effect obscures title bars on open xfce4-terminals and tab labels in chromium.
I don't know if this is an xfce4-panel problem, an xfwm4 problem, or a combination of both. It just started happening recently. I had not changed any of the settings for the top panel from the defaults, nor had I changed the window manager tweaks.