As attached in screenshot
I wrote more details but site keeps crashing so ¯_(ツ)_/¯
When fullscreen editor is open the buttons that I have highlighted don’t work anymore.
This is a small thing, but can really mess with certain workflows
Composition mode is only for document mode (or Scrivenings mode, where multiple documents are edited at the same time), so Corkboard and Outliner modes are unavailable while using Composition Mode. This is working as intended.
I always think it’s marvellously bizarre when people decide how other people should do things. Doubly so when the gatekeeping requires more work than just letting people be.
I’m hesitant to post this because it may be “fixed”, but you can already use any mode you want, you just have to make sure it’s in the correct view before opening up the fullscreen editor.
Why?
Oh, maybe when in fullscreen mode, it’s very hard to keep track of where things are, both hierarchically, and progress-wise. So having the outliner, or corkboard mode open helps. But I’m just a user, so what would I know?
I’m looking forward to having my workflow corrected by your totally infallible personal opinion 
Looking at your screenshot, it seems you have Composite and Full Screen mode loaded on separate displays. I am afraid having the Composition mode open on one screen and Scrivener open on another is not an expected use case. We have not designed for this and some things might not work as expected. You should either use Composite mode or the Scrivener main GUI, but not both. Composite mode is supposed to remove any destructions and leave you alone only with your writing.