1699205 - Scrolling gets stuck intermittently on macOS

Status: NEW

(Core :: Widget: Cocoa, defect, P3)

Description

[joe]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:87.0) Gecko/20100101 Firefox/87.0

Steps to reproduce:

I haven't been able to find definitive steps to reproduce I'm afraid but this is a regular occurence.

I have tried disabling all extensions and gradually re-enabling them in turn to no avail.

Actual results:

I've seen this occur in (at least) two locations.

First, during normal browsing:

1. Open a long web page
2. Start scrolling using a touchpad or magic mouse (which has a sort of track pad built in for scrolling).
3. Scrolling gets stuck so that it's no longer possible to scroll up and down using the trackpad / mouse

You can work around the issue by using other methods of scrolling, including:

1. Spacebar
2. Up/down arrow keys
3. Pg up / down keys
4. Click and drag the scroll bar on the right

Eventually, in some circumstances that again (sorry) I'm not entirely certain about, scrolling behaviour is restored. It feels a little bit like FireFox might be loading some external resource or waiting for a condition on the page before it allows scrolling to happen again, but that's just a suspicion.

I have also seen the same behaviour happen in the web inspector dialog, specifically after opening the inspector and navigating to the Style Editor tab:

1. Attempt to scroll through the list of sources on the left
2. It gets stuck.
3. The same workarounds apply.

I'm on macOS 11.2.3, running FireFox developer edition 87.0b9 (64-bit)

Expected results:

Scrolling should not have got stuck

Comment 1

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Cocoa' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Comment 2

[joe]

If anyone can help by letting me know what to do when the bug occurs to provide better information (maybe there is a debug flag or something?) then please do.

Comment 3

[Martin Balfanz [:mbalfanz]]

I tried to reproduce the bug but had no luck so far. When this occurs the next time, could you share the website that it's happening on? Also, information from about:support could be helpful to see if you have any extra prefs set.

Comment 4

[joe]

Attachment: about:support output

Here's the info I have from `about:support`:

Comment 5

[Timea Zsoldos [:tzsoldos], Desktop QA]

Reproduced with this site: https://dogstudio.co/ on macOS 11.2 with Firefox 89.0b2. Click on "Toggle menu" button, minimize the browser window in order to appear a vertical scroll bar then try to drag and drop the scroll bar, it gets stuck.

Comment 6

[Stephen A Pohl]

(In reply to Timea Zsoldos [:zstimi/tzsoldos], Desktop Release QA from comment #5)

Reproduced with this site: https://dogstudio.co/ on macOS 11.2 with Firefox 89.0b2. Click on "Toggle menu" button, minimize the browser window in order to appear a vertical scroll bar then try to drag and drop the scroll bar, it gets stuck.

I'm able to reproduce behavior that's similar to what is being reported here. When the scrollbar is at the beginning (vertically) of the page, it is intermittently not possible to drag the scrollbar by clicking on it. It behaves as if something was placed on top of the scrollbar thumb in the z-order, but I was not able to identify anything obvious with a quick look at the page's source code.

Comment 7

[Stephen A Pohl]

I can reproduce this as far back as mozregression will take me. Removing the <div class="site-header"> from the page restores the scrollbar's functionality. Possibly because the div itself isn't scrollable?