1540064 - The font's bounding box height should not be affected when zooming in or out the screen

Status: NEW

(Core :: Layout: Text and Fonts, defect, P3)

Description

[Alastor Wu [:alwu]]

This issue seems only happening on Ahem font, which is boardly used in wpt, especially for WebVTT.

STR.

1. Install Ahem font (https://web-platform-tests.org/writing-tests/ahem.html)
2. goto http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/disable_controls_reposition-ref.html
3. Try to zoom in/out with the different ratio

Expect.
4. The height of the bounding boxes keep the same when we apply different zoom ratio

Actual.
4. The height of the bounding boxes are different when we apply different zoom ratio

Note.

• you can check the position of the font box, sometime it fits with the bottom boundary, but sometime doesn't
• the height is changing between 50~51px
• this issue doesn't happen on Chrome
• here is my screen recording https://drive.google.com/open?id=1U_VhXjUVPcMSuJAQ7Yx_ohksffGyVEmM

Comment 1

[Alastor Wu [:alwu]]

Hi, Cameron,
I'm wondering do you have enough bandwidth to help me check this issue?
If not, could you give me some suggestion how should I fix this bug? As it affects most of WebVTT wpt, it definitely need to be fixed if we would like to enable WebVTT rendering tests.
Thank you!

Comment 2

[Emilio Cobos Álvarez (:emilio)]

Seems like the tests should use line-height: 1 or line-height: 50px as well, or such.

Comment 3

[Jonathan Kew [:jfkthame]]

I assume this occurs because of how the ascent/descent metrics of the font get rounded at different sizes or scale factors. Would be nice to make it more consistent, but the simple fix for tests is probably to explicitly set a line-height, as Emilio suggests.