Phil, further testing seems to show that there are significant issues in Safari 7.0.x (Mac OS 10.9) with mouse-event detection and behavior, primarily in the messagelist. Folder list seems less problematic. Things like submenu requiring a second click (not expanding on mouseover)
Clicking to get things like the mark sub menu or the more sub menu to show up is intentional. This is behaviour copied from the core.
orphaned menus hanging out after the action has taken place.
Confirmed in Safari, will fix.
Thanks