2050478 - panel-list submenu opens off screen at certain viewport sizes when accessed via keyboard navigation

Status: NEW

(Toolkit :: UI Widgets, defect)

Description

[Hanna Jones [:hjones]]

Attachment: Screenshot 2026-06-25 at 9.59.15 AM.png

I can reproduce this 100% of the time on MacOS when I am running Firefox at full width. STR:

• make sure Firefox is full width
• navigate to Firefoxview
• use the keyboard to open one of the menus
• use the right arrow key to open the submenu

expected results:

the submenu opens to the left of the main panel so that it's fully visible on screen. This is what happens when you hover over the option/use the cursor to open the submenu.

actual results:

the submenu always opens to the right of the panel meaning the options are always partially obscured

Comment 1

[Hanna Jones [:hjones]]

Attachment: Screen Recording 2026-06-25 at 10.10.42 AM.mov

This seems like a related issue - at slightly smaller browser widths trying to open the menu via the keyboard just causes it to auto close and triggers a strange layout shift/flash