Open Bug 1540064 Opened 5 years ago Updated 2 years ago

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


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





(Reporter: alwu, Unassigned)


(Blocks 1 open bug, )


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


  1. Install Ahem font (
  2. goto
  3. Try to zoom in/out with the different ratio

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

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


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!

Flags: needinfo?(cam)

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

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.

Priority: -- → P3
Blocks: webvtt-wpt
No longer blocks: 1534862
Severity: normal → S3
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