Alt doesn't always get forwarded to the application

I've begun to notice this problem on a fresh Zesty install. I've seen it with gnome-terminal, the standard switcher, and emacs25 but I suspect other applications may have a similar problem.

Hitting an Alt+<key> combination seems fairly racy. Sometimes it will do what I expect but other times the key just gets eaten, with (almost) nothing apparent happening. For example, sometimes when I hit Alt-Tab, I get to the switcher and can happily switch away. Other times, the switcher does not come up until I release Alt+Tab and hit it again. Note that sometimes I might hit and hold Alt just a slight fraction before I hit the Tab key. The amount of time between hitting Alt and Tab is very small but unpredictable.

When I say almost nothing happens when Alt+Tab isn't forwarded to the application, this isn't entirely true. When you're using an application like chromium-browser or emacs25 that's what I see. However, with gnome-terminal, as soon as I hit Alt (but before hitting TAB), I see the application's menus show up in the menu bar. But sometimes, no application menus show up and all you see is "Terminal". I haven't completely correlated this with whether Alt is passed to the application or not but it seems related. I.e. there's weird race condition here somewhere.

I will try to A/B this behavior against a Yakkety desktop, but it does seem like a regression and it makes the desktop difficult to use.