Click "Start" or press ALT-S to open the menu. That's a pretty big menu so it should display scroll arrows. If you hover the scroll arrows, the menu will start scrolling in that direction with 5 pixels every 30 milliseconds. If you press the mouse button, the scrolling speed will double, scrolling 10 pixels every 30 milliseconds.
These values are the defaults and you can get them by passing "scrolling: true" to DynarchMenu.setup. If you need to customize them, you can use for instance the following code:
var scroll_options = {
step1: 10,
step2: 50,
speed: 100
};
DynarchMenu.setup("menu", { scrolling: scroll_options });
The above will make the menu scroll 10 pixels every 100 milliseconds for the normal ("hover") speed and 50 pixels for the high ("mousedown") speed.
Note that keyboard navigation continues to work correctly for scrolling menus--you can press the up/down arrows to select items and the menu will scroll automatically the selected item into view. There's a slight variation for scrolling menus, in that the selection won't "wrap" to the other end of the menu when you reached the first/last item.
The look of the scroll arrows is defined in hmenu.css so they look the same in all skins. It is possible to customize them individually, just look at the /* scroll */ section at the end of the file.