VisualEditor's and OOUI's toolbars are conceptually incompatible. Yet somehow we managed to implement one of top of the other.

Lifecycle of OOUI toolbars:

• (constructor)
• (attached to DOM)
• #initialize
  • #setup
  • #reset
• #destroy

Lifecycle of VisualEditor toolbars (approximate, as each target does it slightly differently):

• (constructor)
• #setup
• (attached to DOM)
• #initialize
  • #detach
  • #setup
  • (attached to DOM)
• #destroy

Problems:

• #setup is called before #initialize. This is just wrong.
• In VisualEditor, each target has one toolbar and multiple surfaces. This necessitates detaching and reattaching the toolbar from surfaces, and thus from the DOM, which is not supported by OOUI (you'd have to #destroy it and #initialize again).
• #initialize is called after the toolbar is attached, but only called once per toolbar. #initialize is explicitly meant to calculate stuff after the toolbar is attached, and the stuff becomes invalid when it is reattached somewhere else. This affects upcoming narrow-display styles for the toolbar (https://gerrit.wikimedia.org/r/#/c/193282/).
• We insert toolbars inside toolbars, which make the #setup and #initialize mess even worse. The actions toolbar was replaced with right-aligned toolgroups. We also (in VE-MW) insert the Save button into the toolbar long after it is attached, initialized and set up, which breaks #initialize's calculations again. Save button is a now a normal tool