Toolbar using client-side session (localStorage) to store the toolbar state now. And this is default to render in horizontal mode. When users toggle to Vertical Mode, on each page load there's a jumping after page is fully loaded. Due to browsers limitations, frontend performance concern and Drupal's design (eg. Can't using inline scripting / Can't delay rendering after JS), we must pre-render CSS class for toolbar in backend.
About `horizontal mode` we fixing in #61, Code explained itself. )
More ref about browser side: http://www.html5rocks.com/en/tutorials/internals/howbrowserswork/
** @droplet: noted that, there's nothing about Backbone performance or bad coding on JS side.
Possible Workarounds (by @droplet):
A: Using Backend session.
B: Delay rendering after Page is fully loaded. (BAD IDEA)
C: Adding Anime to make it looks more smooth. (Needs proof)
D: Drop `horizontal mode` for desktop users.
User interface changes
Data model changes