2004925 - Vertical tabs cause blurry UI and page content

Status: NEW

(Core :: Graphics, defect)

Description

[Martin Balfanz [:mbalfanz]]

Attachment: blur.mp4

Vertical tabs seem to cause blurry pages and Firefox UI. I noticed this first on Slack, when text and image sharpness changed on hover or when scrolling, but could also reproduce this on about:support and in the devtools UI.

STR:

• open Firefox in a clean profile
• visit for example about:support
• switch on vertical tabs
• observe. If you can't see a difference, scroll for a bit or change the window size

ER: text should be sharp

AR: text is blurry

Tested on MacOS 15.7.2 with the latest Nightly (148) on a 4k screen. See video attached

Comment 1

[Alice0775 White]

In Windows 11, enabling vertical tabs causes text to become grayscale AA instead of subpixel AA.

Comment 2

[Martin Balfanz [:mbalfanz]]

I ran mozregression. Unfortunately, there are many skipped builds and the result window is not very narrow: https://hg-edge.mozilla.org/mozilla-central/pushloghtml?fromchange=bde1ea11f25af539acccf38965f2c6c14f4f72d3&tochange=29c957dc0c6f710ef5e2471560a1f49e7c655db9

It was basically when vertical tabs/the new sidebar got introduced. I somehow have a feeling that the issue is related to the rounded corners of the viewport.

Comment 3

[Martin Balfanz [:mbalfanz]]

Attachment: devtools.mp4

Adding a video, showing that devtools is also affected.

Comment 4

[Martin Balfanz [:mbalfanz]]

Did some further digging, and it seems to be related to translate: 0 here: https://searchfox.org/firefox-main/source/browser/themes/shared/sidebar.css#244

When I toggle this off in the browser toolbox, text is sharp again.

Comment 5

[Teodor Tanasoaia [:teoxoy]]

I can't reproduce this on Windows 11 with Firefox Nightly 148.0a1 (2025-12-10). Is it only reproducible on MacOS?
Could you share the content of your about:support?

Comment 6

[Martin Balfanz [:mbalfanz]]

Attachment: about:support

Comment 7

[Martin Balfanz [:mbalfanz]]

Attachment: Screenshot 2025-12-10 at 16.41.19.png

Added my about:support output. I'm testing this on MacOS, can test in Win 11 a bit later.

One more note, which might be relevant: I'm using a scaled display resolution

Comment 8

[Martin Balfanz [:mbalfanz]]

Can't reproduce this on Win11