+ HORIZONTAL_SCROLL_BAR_CLICK_EVENT, /* .code gives the number of the mouse button
+ that was clicked.
+ .modifiers holds the state of the modifier
+ keys.
+ .part is a lisp symbol indicating which
+ part of the scroll bar got clicked.
+ .x gives the distance from the start of the
+ scroll bar of the click; .y gives the total
+ length of the scroll bar.
+ .frame_or_window gives the window
+ whose scroll bar was clicked in.
+ .timestamp gives a timestamp (in
+ milliseconds) for the click. */