Describe the bug

When using transitions and closing the sidebar, the gutter area is not transitioned but removed directly.

To Reproduce

https://stackblitz.com/edit/stackblitz-starters-dyjs7p?file=src%2Fmain.ts

While quite hard to see during the normal 300ms, it becomes a lot easier if you either change the transition time to something longer (I'm guessing here) or if you use DevTools to slow down the animations (see Chrome).

I would expect the gutter area to follow the transition as well, and just be moved "out of view" in the example above.