Namely (it could hardly be different...) Internet Explorer - even IE11 still causes trouble. The default theme now supports IE9+, even older browsers will present broken markup. The sliding animation works with IE10+ (however, it is still usable, there's just no nice animation).
Furthermore this commit heavily improves the sliding process by allowing to abort the animation. I've updated Pico's screenshot in the , too.
This reverts commit a3fa373119.
At first glance this adds flexibility, but at the moment it is impossible with Twig to ensure the existance of a block. As a result, custom themes may break the plugin. A custom theme should overwrite a plugin's template explicitly.