Aahhh, thanks so much for posting this. I spent way too much time this afternoon trying to figure out where my missing left panel went. I hate the collapsed view because I switch between notebooks often and I'd rather click once than twice. Anyway, I have multiple mis-matched monitors. I have a 4k monitor scaled up to 200% so I can see text. I regularly switch the 4k down to 1920x1080 to see older applications that won't scale, but look better on a wide screen. My 2nd monitor tops out at 1280x1024. This morning, I had the 4k down to 1920x1080 and had to switch it back up to 3840x2160 (because of a web conference sharing bug, but that's another story). I had EN open at the time (on the small 2nd monitor) and noticed a little later that my left panel was minimized and I could not get it to expand. It was minimized on both monitors.
Hours of searching and frustration.
Then I ran across this thread and tried switching my resolution back to 1920x1080 (where it was this morning).
My EN left panel reappeared! F10 worked again!
Then out of curiosity, I switched back up to 3480x2160 and the left panel stayed put. Soooo, it does appear to be linked to display resolution somehow.