GLUT_BORDERLESS seems to vary by the window manager on X11, though twm (for example) performs very similarly to WIN32. But KDE's window manager (for example) does not let you send keystrokes to borderless windows without OpenGLUT hacks.
Mice are optional, but OpenGLUT is only able to check for them under WIN32.
Mice can have a varying number of buttons, but OpenGLUT assumes exactly 3 on UNIX_X11.
Not all joystick queries are implemented yet.
GLUT_DEVICE_KEY_REPEAT returns the key repeat mode, but the comment says it returns the rate.
Some things, like joystick poll rates, seem to have insufficient context. Which joystick? Which window? Maybe we assume the current window and the current joystick (or the first one)?
GLUT_DEVICE_KEY_REPEAT should probably return ogState.KeyRepeat.
Does not differentiate between the left and right forms of the modifiers.
The following returns an unsigned long, but the function is only set to return {int}.