In Lightroom 4.3 on OS X 10.8 for me, hovering the mouse pointer over the left edge of the library fails to temporarily reveal the panel with navigator/collections/publish services. I have to click on the triangle to permanently show it then click the triangle again to hide it.
@Dorin this sounds promising -- I wasn't aware there was a popup menu here or that this was configurable. But when I right-click to show the pop-up, all I get are options for displaying the various panel sections (navigator, catalog, etc.), then Show all, Hide All, Solo Mode, Expand All, Collapse All, and Panel End Mark. Am I looking in the right place for this pop-up?