library_glfw.js: GLFW_RELEASE not sent for keys after cmd/meta/menu key is pressed on mac

**Version of emscripten/emsdk:**
3.1.15

I'm using glfw (via Dear ImGui) and if I press cmd+c and let go of the c and then the cmd, I never receive the GLFW_RELEASE event.

I added the following print to library_glfw.js
```
onKeyup: function(event) {
console.log("onKeyChanged keyup: " + event.keyCode + "\n");
GLFW.onKeyChanged(event.keyCode, 0); // GLFW_RELEASE
},
```

That informed me the onKeyUp never gets fired. Searching around on the net, I see some discussion in other forms such as (https://github.com/electron/electron/issues/5188, https://stackoverflow.com/questions/11818637/why-does-javascript-drop-keyup-events-when-the-metakey-is-pressed-on-mac-browser) which indicate that the Javascript onKeyup never gets called in cases where the browser takes over (such as the cmd+c/v/etc cases on mac).

I'm not familiar enough with the web platform to know if that's a browser bug or not. I imagine it's probably underspecified by the standard. Most everyone has accepted it's just the way it is and deal with it.

However, filing this bug to ask: should users of GLFW on emscripten be expected to deal with it? Presumably GLFW is supposed to abstract the underlying platform and thus should mitigate this somehow?