1351080 - [e10s] <select> drop-down doesn't appear near <select> if CSS 'transform' is applied to iframe

Status: NEW

(Core :: Layout: Form Controls, defect, P3)

Description

[684sigma]

Attachment: drop-down doesn't appear near select, by reporter.html

I have a problem with Firefox Beta 52. It also happens in Beta 53, Nightly 55. Doesn't happen in ESR 45. (Well, I remember that it happened in some other cases, but in this exact case there's no bug in ESR 45. I'm not sure whether this is considered as regression)
Sometimes drop-down list of <select> options appears far away from <select> if CSS 'transform' is applied.
Here's how to reproduce it:

1. Open attached document
2. Click on <select>

Result: drop-down appears in a distance
Expected: drop-down list should be exactly below the <select>

I've added "[e10s]" in the summary, because it isn't reproducible if in Nightlly I disable multiprocess.
Please correct me if this logic is incorrect.

Comment 1

[684sigma]

* not sure

Comment 2

[Loic]

Attachment: screenshot-FF52-with-e10s.jpg

Comment 3

[Astley Chen (inactive)]

NG on Windows and GNU/Linux build but works on macOS.
Nothing shows up on Edge and Chrome when loading test case in this bug.