Nightly: 77.0a1 (2020-04-24) (64-Bit)
macOS: 10.15.4

Steps to reproduce:
1.) Enable the default setting in System Preference "Show Scrollbar automatically based on moused or trackpad"
2.) Open a page where you can scroll
3) Scroll a bit down so that scrollbar appears during scroll and disappears again when scrolling stops
4) Now scroll to the very top again

Actual: The scrollbar does not disappear again.

Expected: When you reach the very top/left/right or bottom of the page, the scrollbar should disappear again.

This is a regression at least in recent Nightly.
This is not reproducible in Firefox Stable 75.

A screencast of the issue is attached.

Thanks for fixing it in advance!

Hello,

I did a manual bisect and I found the following regression range:

Good build: 2020-04-06 21:47:50 (mozilla-central)
Bad build: 2020-04-07 09:35:20 (mozilla-central)

Not sure if I am doing it right, but when I check the pushlog between these times, I receive the following output: https://hg.mozilla.org/integration/autoland/pushloghtml?startdate=2020-04-06+21%3A47%3A50&enddate=2020-04-07+09%3A35%3A20

I see that echen@ (Edgar Chen) did some changes on mouse behavior in that time.

@Edgar Chen: Can you please check, if this regression could be caused by one of your changes or help to find the culprit?

Many thanks in advance :)

I could also reproduce on the latest Nightly, but it doesn't look like something related to bug 1615732, bug 1615732 should only affect click + ctrl, but I will take a look at anyway.

12:33.33 INFO: Last good revision: 76afd03a4c036a99e1c93cd17d2b71447c837f85
12:33.33 INFO: First bad revision: 6aec54d11cbf34fe8f157c1d982a45ef872416e9
12:33.33 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=76afd03a4c036a99e1c93cd17d2b71447c837f85&tochange=6aec54d11cbf34fe8f157c1d982a45ef872416e9

It looks like a regression of bug 1589046. Kartikaya, could you take a look? Thanks.

Thanks, I'll take a look. Sounds like we're probably not sending all the right notifications over to the main thread.

https://hg.mozilla.org/mozilla-central/rev/7e72af7cd1f6

Thanks for the fix!

Confirmed issue with 77.0a1 (2020-04-24) on macOS 10.15.5.
Fix confirmed with 77.0.1 using the touchpad.