Yes, indeed, it cause by compositor part in xfwm4, all I have to do is just uncheck the "show shadows under dock windows" under the Compositor tab in "Window Manager Tweaks" of xfce4-settings-manager, and they are gone now, thanks, they are annoying me for months.
Yes, indeed, it cause by compositor part in xfwm4, all I have to do is just uncheck the "show shadows under dock windows" under the Compositor tab in "Window Manager Tweaks" of xfce4-settings- manager, and they are gone now, thanks, they are annoying me for months.