Answer
The target briefly highlights. On updated builds, Hashchats can gently adjust its position for up to five seconds as images, videos or link previews finish loading. Scrolling yourself, choosing another target or resizing stops that adjustment. Reduced motion makes it immediate.
Messages taller than the available space still need scrolling. Loading more history preserves the message you are reading while the loaded window changes. Continue toward older or newer messages, or use Latest to return to the newest ones.
This applies in private and People chats on web and native mobile. Older installed builds may differ.