sticky-admin-sidebar.js

function sticky_admin_sidebars() {
	// Define area to scroll and what it's relative to
	var sidebar_container = '#postbox-container-1';
	var parent_container = '#poststuff';

	// Determine heights
	var sidebar_height = jQuery(sidebar_container).height();
	var parent_height = jQuery(parent_container).height();

	//  Get offsets of document and sidebar starting position
	var document_offset = jQuery(window).scrollTop();
	var initial_offset = jQuery(parent_container).offset().top;

	// Add extra offset when scrolling so looks nicer and set default sidebar offset
	var extra_offset = jQuery('#wpadminbar').height() + 10;
	var sidebar_offset = 0;

	// Work out maximum scroll amount by subtracting the sidebar height from the height of the parent
	var max_scroll = parent_height - sidebar_height;

	// If sidebar height is more than the parent's height or sidebar is not shown on the sidebar, set position to static and stop
	if (parent_height <= sidebar_height || jQuery(window).width() < 850) {
		jQuery(sidebar_container).css('position', 'static');
		return;
	}

	// If we have scrolled past the initial offset
	if (document_offset > initial_offset) {
		// Determine sidebar offset
		sidebar_offset = document_offset - initial_offset + extra_offset;

		// If the sidebar offset is more than max_scroll, set to max_scroll
		if (sidebar_offset > max_scroll) {
			sidebar_offset = max_scroll;
		}
	} else {
		// Sidebar hasn't past initial offset or user has scrolled back to top
		sidebar_offset = 0;
	}

	// Set position to relative and animate into view
	jQuery(sidebar_container).css('position', 'relative').animate({
		top: sidebar_offset
	}, 250, 'swing');
}

// Monitor scroll event
jQuery(window).scroll(function() {
	// Only run positioning function when user stops scrolling
	// Thanks to @yckart - http://stackoverflow.com/a/14092859/236038
	clearTimeout(jQuery.data(this, 'scrollTimer'));
    jQuery.data(this, 'scrollTimer', setTimeout(function() {
        sticky_admin_sidebars();
    }, 250));
});

// If window has resized or loaded for the first time run positioning function
jQuery(window).bind('resize load', function() {
	sticky_admin_sidebars();
});