> Sometimes, keyboard shortcuts seem to be executed twice!

Sounds like someone made a listener that listens on key events, but didn't bother to check the state of the event, meaning it hits releases as well. Should be easy to verify by keeping them pressed long enough to trigger the key repeat events.

> I also noticed that font rendering is different between X11 and Wayland! The difference is visible in Chrome browser tab titles and the URL bar, for example:

Tab title display is not owned by wayland unless you are running with the client side decor extension, which Gnome is not. So looking at the application or GUI framework (GTK in this case) are really the two only choices.