Affected versions

• 80.0a1 (2020-07-02)
• 79.0b3
• 78.0

Affected platforms

• Windows 10
• Ubuntu 18.04
• macOS 10.15

Steps to reproduce

1. Resize the browser’s width below 1000px
2. Open https://sale.aliexpress.com/__pc/XZL7sThtbD.htm?spm=a2g0o.home.15001.3.650c21458d8EvY&productIds=33023056108

Actual result

• Bad painting on top of the page

Expected result

• No painting issues are visible

Regression range

• I will provide additional information asap

Additional notes

• This seems like site issue at its root, but note that on Chrome the issue is barely visible, while on Firefox the bad repainting stands out
• More noticeable on Windows and macOS

This is a site issue. When the window is narrow, the orange bar of "flash deals" categories becomes horizontally scrollable, and the "bad painting" we see in Firefox is the scrollbar area being blanked.

This doesn't happen in Chrome because they're using the non-standard ::-webkit-scrollbar pseudoelement in flash-deals.css to suppress it:

::-webkit-scrollbar {
    display: none;
    width: 0;
    height: 0;
}

This is not interoperable and should not be relied on; see https://developer.mozilla.org/en-US/docs/Web/CSS/::-webkit-scrollbar.

Adding the (standards-track, though not yet implemented by other browsers) property scrollbar-width: none to the element would fix the display in Firefox. See https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-width.

according to Bug 1432935
This can be fixed with the system set into Bug 1561299

I guess this is a yes.
https://github.com/mozilla-extensions/webcompat-addon/blob/27e6c5a448c7683df03721d0a38b6fdc0a389a7e/src/data/injections.js#L208-L222

Should we move this bug to site intervention or should we clone it into a new bug?
I wonder what was the previous process for this.

I've created a new bug and linked it to v13.0.0 tracking bug of system addon