I just installed Debian 9 (Stretch) this morning and I can't get my Meta key working. It used to work completely fine in Debian 8 (Jessie) and I don't remember having any problem.
The X server seems not to see the key at all. xev sees nothing at all when pressing the Meta key. All other keys on my keyboard work fine. The keyboard is a Logitech G710+ ; setxkbmap -print -verbose 10 says rules are evdev, and model is pc105, which is fine as far as I can tell.
I made sure the desktop environment I'm running (KDE5) does not interfere in the following way : I've downed the entire X environment, logged in on the text console, ran X :0 and a term in :0, from where I've run xev without even a window manager running, but the result is the same : the Meta key alone is not seen by xev.
So far I've always used xev to figure out where keys are getting blocked, but this time it does not even see anything happening and my internet searches have not turned up anything I could use to figure out what's wrong.
What would be the next step for me to investigate what's happening ?