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

Status: RESOLVED FIXED

(Toolkit :: UI Widgets, defect, P2)

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

Comment 2

[Tim Giles [:tgiles]]

Might be related to the popover API changes, I need to confirm one way or the other.

Comment 3

[Tim Giles [:tgiles]]

Attachment: Bug 2050478 - Part 1: Fix panel-list submenu auto-closing when opened by keyboard. r?#recomp-reviewers

When a submenu was opened with the keyboard at viewport sizes where the
page can scroll horizontally, it would flash open and immediately close.
The submenu's ArrowRight/ArrowLeft handler did not preventDefault, so the
arrow key would scroll the page and then the scroll-hide listener would
dismiss the menu.

We add preventDefault to the submenu open and close key branches, since
this is already done for ArrowDown/ArrowUp/Tab.

Includes regression tests for the arrow-key scroll dismissal in both LTR
and RTL, where the open and close keys are swapped.

Comment 4

[Tim Giles [:tgiles]]

Attachment: Bug 2050478 - Fix two panel-list submenu defects on keyboard open: dismissal on page scroll and off-screen opening. r?#recomp-reviewers

A submenu opened by keyboard near the right viewport edge could open
off-screen. setSubmenuAlign() measured the submenu bounds in a single
requestAnimationFrame, but on the keyboard path the submenu content was
not laid out yet, so its width read as 0 and the overflow check chose the
wrong side. This race is timing-dependent and surfaced on macOS but not
Windows, though measuring before layout is wrong regardless.

We defer the measurement with requestAnimationFrame + setTimeout to match
setAlign(), and replace the align-attribute heuristic with an explicit
check of the space available on each side so the submenu opens where it
actually fits. topOffset is also rounded to avoid sub-pixel artifacts,
matching setAlign().

Includes regression tests for the keyboard submenu positioning in both
LTR and RTL.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/803931c078ca
https://hg.mozilla.org/integration/autoland/rev/917e73d8ce07
Fix two panel-list submenu defects on keyboard open: dismissal on page scroll and off-screen opening. r=hjones

Comment 6

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/917e73d8ce07