2020917 - canvas measureText() resolves system-ui / -apple-system to a different font than DOM on macOS

Status: UNCONFIRMED

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

Description

[chenglou92]

Attachment: system-ui-repro.html

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.3 Safari/605.1.15

Steps to reproduce:

1. Open the attached system-ui-repro.html in Firefox on macOS
2. Observe the table comparing canvas measureText width vs DOM span.getBoundingClientRect() width

Actual results:

16px system-ui:        canvas=375.2  dom=423.5  diff=-48.4  MISMATCH
16px -apple-system:    canvas=375.2  dom=423.5  diff=-48.4  MISMATCH
16px sans-serif:       canvas=410.1  dom=410.1  diff=0.0    OK
16px "Helvetica Neue": canvas=414.4  dom=414.4  diff=0.0    OK
16px Georgia:          canvas=406.4  dom=406.4  diff=0.0    OK

Expected results:

Canvas and DOM return the same width for the same font string. (Chrome and Safari do this correctly.)

Additional observations

• Only system-ui and -apple-system are affected — all named fonts and other generic families match perfectly
• The difference is ~48px on a ~400px string (12% error)
• Chrome and Safari canvas/DOM agree for system-ui on macOS
• Both regular <canvas> and OffscreenCanvas are affected

Impact

Canvas-based text measurement libraries that predict line breaks cannot use system-ui or -apple-system on Firefox macOS. The canvas measurements are too far off from DOM rendering to produce correct results.

Comment 1

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'Core::Graphics: Canvas2D' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 2

[Jim Mathies [:jimm]]

Lower priority for now. I'm not sure if this is in canvas or with fonts, Lee, care to comment?

Comment 3

[Lee Salzman [:lsalzman]]

Jonathan, ideas?

Comment 4

[BugBot [:suhaib / :marco]]

The component has been changed since the backlog priority was decided, so we're resetting it.
For more information, please visit BugBot documentation.