To reproduce:
1. Run /help
to fill up the chat
2. Open chat
3. Very very slowly scroll up (move the mouse scroll wheel by one tick, wait a second, repeat)
4. Observe chat not scroll up
Scrolling up quicker and scrolling down at any speed work.
I am using a mouse with a physical scroll wheel, I tested two different mice of this kind. Attached is a forced crash report.
Note that the reproduction steps produce expected behavior in the creative mode inventory, proving that this is not caused by my hardware.
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Was only able to reproduce once but can confirm.

Cannot reproduce for 1.13.1, might be my mouse's scroll wheel.
@unknown Could you try to reproduce this? thank you 🙂

I Can't repro what the reporter is describing. I mean, the chat scrolls in chunks but thats wai.Â

Can still reproduce in 20w08a. Could be considered WF since this doesn't happen with the "Discrete Scrolling" option that was introduced after this report.

Created related issue MC-172989 which may have the same root cause.

Turns out the root cause is the same, see @@unknown's code analysis on the other ticket. Forward-resolving as duplicate.