What actually happens
ChatGPT only keeps part of a long conversation in the page at once and loads more as you scroll. When you print, the browser captures what is currently laid out, so messages that were not rendered get clipped. The chat interface is also styled for the screen, not for paper, which adds awkward breaks.
Why scrolling first is not enough
Scrolling to the top can load more messages, but very long chats still break unpredictably, and dark backgrounds print poorly. You end up fighting the layout instead of getting a clean copy.
The quick fix
CHATGPT.Vellum saves the whole conversation as a clean PDF in one click, built inside your browser. Print a ChatGPT chat in full.