alt Hacker NewsIf you mean that you push a 18650 inside the steerer tube and have the display at the cap level - how do you preload the headset then? You need to pull the steerer up somehow and it's done via that cap and a bolt that goes through it into a compression plug/star nut inside the steerer. If you place something under the cap it needs to fit between that bolt and the inside of the steerer tube, which 18650 can't.
Some modern headset designs don't use the top cap to do that. There's a bunch of products that exploit that to put toolkits, spare CO2 canisters or whathave you in that spot.