The preview was done quickly and without thinking, so it's not the most logical and beautiful. But it is, and it seems to me that it illustrates how the component should behave.
This item is partially designed. In addition to the "general" button in the navbar, in the general list and microblog as well as in the content belonging to the selected channel, a form is visible at the top. When clicked, it should go to the full, expanded form. You can also do as on #reddit, i.e. move to a new tab after clicking, but in my opinion this is a bad solution and unintuitive.