There're many reports of users trying to use Wayland and just seeing a black screen. Since I was one of these users and I found the solution on Bug 456337, here is a suggestion to improve the situation.

In the src/main_wayland.cpp, the automaticBackendSelection() will select the backend based on WAYLAND_DISPLAY or DISPLAY environment variables. So, if the user sets the DISPLAY variable, automaticBackendSelection() will use the X11 backend (and never use DRM backend), so the user will be stuck with a black screen:

static QString automaticBackendSelection()
{
    if (qEnvironmentVariableIsSet("WAYLAND_DISPLAY")) {
        return s_waylandPlugin;
    }
    if (qEnvironmentVariableIsSet("DISPLAY")) {
        return s_x11Plugin;
    }
    return s_drmPlugin;
}

It would be nice if it could be more verbose, like "DISPLAY environment variable detected. Using X11 backend" or "WAYLAND_DISPLAY environment detected. Using Wayland backend". So the log will help the user to understand that if (s)he wants to use the DRM backend, (s)he cannot set those environment variables.

Thank you!