How to edit a card description in full screen?
To enable the full-screen mode for the card description, please focus your mouse cursor on the description field, as if you wanted to start typing in it. You will see a stretch arrow in the top corner of the description field, which activates the full-screen view. To get back to the regular view, just hit the ESC button. You can also use keyboard shortcuts within the task description field, please see them here.