If you drag the composer bar up to its fullest extent, it becomes impossible to drag it back down again.
The header overlaps the ‘grab bar’ of the composer, making it ungrabbable. The only way to get the composer back to where it should be is to resize the window (if you can) or resize the text. This is a bit of a pain, and beyond the skills of many users!
This is in Chrome on Desktop (Mobile is fine). Sorry I haven’t tested it more widely.