I tried the new global menu. I figured it out and it worked. So a created a new empty panel in the top of my screen and put the widget in it. I made this panel "autohide". It seemed an elegant thought: when you don't need the global menu it isn't there. But the autohide function was at the least very unreliable. Most of the time the panel is just visible. It stays always on top. I use plasma 5.9 on kde neon
I can't reproduce. I just added an global menu applet to a panel and it auto-hides correctly. Do you have the panel maximized (ie. spans the full width of the screen)?
No, I centered it and made it as small as I could but gave it all the space to expand when there are many items in the menu.
Kwin unsets auto hide state when the geometry of the panel changes. (There's a bug report about that bug I can't find it right now)
Found it :) Bug 373958 *** This bug has been marked as a duplicate of bug 373958 ***