Sitemap navigation has dramatically changed in CRM 2013 with horizontal layout replacing classical sitemap tree of CRM 2011. If group contains large number of subareas, horizontal scrolling is added. Large and shiny 24″+ monitors with high resolutions just compound the issue if you need to get to the rightmost item in a large group. Mouse pointer all the way to the right, then click on a tiny horizontal scroll arrow, then all the way back to the required item that managed to scroll past the pointer.
I trust that your mouse does have a scrolling wheel that you’ve been using to scroll web pages and documents. Turns out, the very same wheel will scroll the sitemap group right and left as you rotate the wheel down and up. Move mouse over the group name (e.g. Sales), group will expand (no click required), move the mouse pointer just few pixels down so that it’s within the group boundaries and when rotate the wheel. See?