I have done this many times:
1. I click "New post"
2. I type a post
3. Now that the post is done, I see it needs a content warning. I click the content warning button. I click in the content warning entry field. I type a content warning message.
4. I click "Post Now".
It DOESN'T WORK!! There is no content warning on my post and now everyone on my feed is looking at giant photos of adorable spiders until I notice it and scramble to edit. What I SHOULD have done is enter a step 3.5: Press "Tab". But I don't press "Tab" because there's no reason to press "Tab", why would I think to do that, and also if I am on a phone I have no "Tab" key.
This UI is especially misleading because you can insert many steps between 3 and 4, for example you can click to move cursor focus back to the post and type in the post area, in fact you can write the content warning *first* and the content warning will do *nothing* unless the magic tab key is pressed in the field to "accept" it. There is no UI feedback that the entered content warning is provisional rather than accepted.
EXPECTED BEHAVIOR [BASIC]: Content Warning is an important thing and you don't want to accidentally lose it. If Content Warning is entered but not "accepted" and the user clicks "Post Now" or "Save Draft" a popup box should appear like "use content warning [blah]?".
EXPECTED BEHAVIOR [IDEAL AND POSSIBLY LESS WORK]: Entering something in the content warning field and then removing focus from content warning (for example clicking in the basic post entry area, or clicking "post now") should accept the content warning.