We improved the AltGr handling in bug 5226 for the native client. Similar work was done upstream for noVNC:
And got merged in to Web Access via bug 6152.
However Ctrl+AltGr is still broken in IE and Edge, so there is some detail missing.
The reason this works in Firefox and Chrome is because they do the AltGr detection for us and send 'AltGraph' as the key. IE and Edge aren't as friendly and send 'Alt' instead.
TigerVNC solves this by examining the keyboard layout. Unfortunately we cannot do the same here as there is no API to do that.
Let's close this bug as there isn't anything more we can do at the moment, and hopefully this corner case will not affect any real users as it is doesn't affect Firefox or Chrome.
This was fully tested on bug 6152, so closing.