In checking out some strange behaviour when docking blocks in Moodle 2.4 ALPHA I found that the Serenity theme dock was not working correctly. Serenity, like Standard theme, uses the layouts from it's parent theme which in this case is Canvas.
On closer inspection of Canvas, I found that the layout files, frontpage.php and general.php, were missing some PHP code namely $showsidepre and $showsidepost. These are needed to extend the dock to enable it to work correctly. It could actually be contributing to the behaviour in Serenity, as well as other themes who's parent is Canvas.
There are also some redundant settings in canvas/config.php where parent exclude stylesheets do not exist any longer, so these need removing.
It has been pointed out on a few occasions, that the dock should be enabled in Canvas and also the custommenu in order to see that Canvas works correctly with these two features, if it is to continue to be a parent theme. Some of the themes that use Canvas as a parent exclude the pagelayout.css, so the only things they get from Canvas are the stylesheets, which in most cases are over written with theme styles anyway.