The system was lost with the editor refactoring which came with saas-10
and commit bf39884a5c.
Indeed the show() method of the BuildingBlock class was removed as the
widget is now always showed but the show_blocks() method it called was
not removed (and thus was not called anymore).
As a fix, the show_blocks method is now called again, on snippets
initializations and each time a snippet is dropped or removed.