panels don't take account of xrandr expansions

[rdtennent]

I "expand" my MATE desktop by using

xrandr --fb 3840x2160 --output DP-1 --mode 2560x1440 --panning 3840x2160

But the panels don't expand or move accordingly. For example, there should be a way to expand the top-edge panel from 2560 to 3840 or to move the bottom-edge panel to 2160 from 1440.