get-display-left-top-inset in Global Graphics and Screen
Returns the amount space at the left and top of the screen that is
occupied by the task bar (Windows) or menu bar (Mac OS). Under
X, the result is always 0 and 0.
Under Windows, if the task bar is bottom-aligned or right-aligned, the
result of this function will be 0 and 0. The task bar's
size can still be determined by comparing the results of
get-display-size with #t and #f.