1407690 - Stylo: Fonts declared in ex appear larger than expected

Status: RESOLVED DUPLICATE of bug 1391341

(Core :: CSS Parsing and Computation, defect)

Description

[Alexander Ploumistos]

User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0
Build ID: 20171009125822

Steps to reproduce:

After installing Firefox 57 Beta in Fedora 26, I visited several Fedora Wiki pages, e.g.:

https://fedoraproject.org/wiki/QA:Updates_Testing
https://fedoraproject.org/wiki/Updates_Policy
https://fedoraproject.org/wiki/Communicating_and_getting_help

And here are some screenshots of the same pages, in case you can't reproduce the issue:
https://alexpl.fedorapeople.org/screenshots/Screenshot-2017-10-11%20QA%20Updates%20Testing%20-%20FedoraProject.png
https://alexpl.fedorapeople.org/screenshots/Screenshot-2017-10-11%20Updates%20Policy%20-%20FedoraProject.png
https://alexpl.fedorapeople.org/screenshots/Screenshot-2017-10-11%20Communicating%20and%20getting%20help%20-%20FedoraProject.png

On the computers I tested (all running Fedora 26) these pieces of text are displayed using DejaVu Sans Mono. As I don't think I fully grasp how the ex units should behave, I am not sure if Stylo is to blame or if Stylo has exposed another bug or faulty implementation.


Actual results:

I noticed that all the text with font size declared in ex, appeared larger than before. I checked the same pages with Seamonkey and Chromium and there was no such issue.



Expected results:

Perhaps the text should be smaller?

Comment 1

[Jonathan Kew [:jfkthame]]

Looking at the https://fedoraproject.org/wiki/QA:Updates_Testing page (using Nightly on macOS) for example, I can confirm that I see a font sizing difference between stylo-enabled and stylo-disabled. So there's something here that needs to be investigated.

Comment 2

[Jonathan Kew [:jfkthame]]

Reduced testcase:

  data:text/html,<div style="font-size:xx-large">one <tt style="font-size:1.5ex">two

With stylo DISabled, the computed font-size of "two" is 18.0333px. With stylo ENabled, it's 22.2px.

This doesn't happen if the parent <div> has an absolute font-size:

  data:text/html,<div style="font-size:32px">one <tt style="font-size:1.5ex">two

In this case, both stylo and non-stylo result in 22.2px.

I think this is related to the complex inheritance of default font sizes, etc.

Comment 3

[Jonathan Kew [:jfkthame]]

Similar behavior occurs with the 'ch' unit, too.

Comment 4

[Jonathan Kew [:jfkthame]]

Actually, I think this is a dupe of bug 1391341.