2050182 - [Nova] [Vertical tabs] [Sidebar] Double line displayed between open tabs and Tools

Status: VERIFIED FIXED

(Firefox :: Sidebar, defect, P2)

Description

[Camelia Badau [:cbadau], Desktop Test Engineering]

Attachment: issueLine.png

Found in

• Nightly 154.0a1

Affected versions

• Nightly 154.0a1

Tested platforms

• Affected platforms: Windows 11, Ubuntu, macOS 13 ARM

Preconditions

• the browser.nova.enabled pref is set to "true"
• the Vertical tabs option is enabled
• have the Sidebar opened in Narrow mode
• have some Tabs and Pinned tabs available

Steps to reproduce

1. Observe the separator line between open tabs and Tools.

Expected result

• The separator line between open tabs and Tools is a single line, consistent with the one between Pinned tabs and open tabs.

Actual result

• The separator line between open tabs and Tools is rendered as a double line. Please see the attachment.

Regression range

• Not applicable.

Comment 1

[Sarah Clements [:sclements] | PTO until Oct 12th]

I'm not seeing this in the latest Nightly. Please reopen if needed.

Comment 2

[Petruta Horea [:phorea]]

Attachment: sidebar tools and extensions splitter

Comment 3

[Petruta Horea [:phorea]]

Issue is still reproducible using Nightly 155.0a1 2026-07-23 under Win 11 and Ubuntu 24.
macOS 15 platform is no longer affected.

Comment 4

[Alexandru Trif, Desktop Test Engineering [:atrif]]

Attachment: image.png

In the collapsed state, there is also some shadowing between open tabs and the Tools divider.

Comment 5

[Kelly Cochrane [:kcochrane]]

Attachment: Bug 2050182 - Prevent default splitter background-color on Win/Linux for the sidebar tools/extensions splitter

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/e6a9a2187951
https://hg.mozilla.org/integration/autoland/rev/54fb1cf6d159
Prevent default splitter background-color on Win/Linux for the sidebar tools/extensions splitter r=nsharpley

Comment 7

[amarc]

https://hg.mozilla.org/mozilla-central/rev/54fb1cf6d159

Comment 8

[BugBot [:suhaib / :marco]]

Since nightly and release are affected, beta will likely be affected too.
For more information, please visit BugBot documentation.

Comment 9

[BugBot [:suhaib / :marco]]

The patch landed in nightly and beta is affected.
:kcochrane, is this bug important enough to require an uplift?

• If yes, please nominate the patch for beta approval.
  • See https://wiki.mozilla.org/Release_Management/Requesting_an_Uplift for documentation on how to request an uplift.
• If no, please set status-firefox156 to wontfix.

For more information, please visit BugBot documentation.

Comment 10

[Camelia Badau [:cbadau], Desktop Test Engineering]

Verified as fixed on Windows 11, macOS13 and Ubuntu 22 using the latest Nightly 157.0a1 (2026-09-08): no double line is displayed, and there is no shadowing between open tabs and Tools.