MOSFET amplifier biasing and headroom?

Okay so my question relates to biasing and threshold voltage in a MOSFET amplifier. So in an amplifier the clipping occurs when the signal hits the power rails according to all the reading I’ve done. That’s how much voltage swing you supposedly have before clipping. So if you have an 18 volt supply you should have +/- 18 volts of headroom. But clipping also occurs if the sine wave goes outside of the saturation region of the MOSFET. If the threshold voltage is say 5V, and you bias is halfway between the threshold and the supply, then you don’t have +/- 18 volts of available swing before clipping, you have much less. DC biasing the transistor obviously turns it on so the 0V AC is superimposed on a suitable voltage to get it over the threshold, but then your voltage is just closer to the supply rail and you have less headroom in the other direction, right? I mean, unless I’m totally wrong about how headroom relates to the threshold voltage (and please god tell me I am,) then why would this not be brought up in any readings on transistor biasing and amplifier design?