VNC protocol transfers key symbols (= basically characters), not key codes (= "coordinates" on keyboard). Therefore pressing the same keys may result in sending different commands over VNC depending on the keyboard layout and state of modifiers on the client side. The server however can not directly send the key symbol to the application, it must instead find or create key code that will translate to that symbol and send that.

This is hard to implement correctly and there was already several bugs about it.

I want to create a testing tool that will open xev-like window inside a VNC session and also connect to the X server where a vncviewer connected to the VNC sessions runs. Then it will send all possible combinations of keys and states to the vncviewer and check if the testing window receives the right events.