I'm on Logic Pro X 10.4.7 and have similar problem of large grey area on the left of my mixer (wasting space on my monitor). Sometimes it's just a matter using left-right scrolling to locate it again. Other times it fails to re-appear, however if I drag the mixer wider and narrower (pulling the right-hand edge of mixer in and out like a squeeze-box!) it reappears!
Realise this may well be a well known glitch - but it motivated me to join your forum: Hi Y'all!
steve