2059664 - Odd placement for X / Close button for installed extensions in Sidebar

Status: NEW

(Firefox :: Sidebar, defect, P2)

Description

[Catalin Sasca, Desktop Test Engineering [:csasca]]

Found in

• 155.0a1 (2026-07-30)

Affected versions

• 154.0b3
• 155.0a1 (2026-07-30)

Tested platforms

• Affected platforms: Ubuntu 26, Windows 11, macOS 26

Preconditions

• Ensure that sidebar is enabled
• Have at least one extension installed (ex. Claude in Sidebar)

Steps to reproduce

1. Open Sidebar
2. Select the installed extension

Expected result

• The Close button has the same position as in the Tabs from other devices or Open History for example

Actual result

• The close button jumps near the upper edge

Additional notes

• The issue can be seen in the following attachment
• This affects Nova styling as well.

Comment 1

[Sam Foster [:sfoster] (he/him)]

This will reproduce with any extension that has a sidebar action. The panel content is loaded into a xul:window wrapped in a webext-panels.xhtml document.
I tracked the discrepancy down to the use of the --sidebar-panel-header-heading-margin-block variable in sidebar.css. This is not defined in the webext-panels.xhtml document. I guess we need to add it to webext-panels.css