This is a minor cosmetic thing.
When I'm writing a post on the desktop site and want to insert an emoji I click on the emoji button and the box pops up. To hide it again I have to click the emoji button again. It's not a big deal having it there but sometimes if the post I'm writing is quite long then I have to actually scroll up my screen to find the button to click to hide the box. This wouldn't be an issue at all except the box sometimes blocks some of the text in my post as you can see in the screenshot below. Perhaps a little 'x' or clicking outside the box could be implemented to hide it?