Upon clicking on a section, it becomes highlighted, and its blocks will stay on screen, clicking on “more” gives a second set of categories which may contain: the rest of the extensions, “obsolete blocks” categories, Labs categories, recent and favorite sections, and of course a back button (in case you want to go back to the main section). Maybe you could have the block palet appear on the left side when hovering over a section on the left file, so you don't have to click the section every time.
