[M24] Submenu hidden under nav on mobile

[janbrasna]

Description

In mobile layouts (portrait and landscape), pages with submenu have the subnav component hidden under the new masthead:

Steps to reproduce

Open https://www-dev.allizom.org/en-US/firefox/new/ in mobile viewports.

(Reproduces in desktop Responsive Design Mode too.)

Expected result

Submenu reachable, placed after the main nav.

Actual result

Submenu hidden under the nav:

RPReplay_Final1726589621.mp4

Environment

Mobile Safari 15–17, FxiOS (+Focus iOS)

[alexgibson]

Possible duplicate issue: #15214

[wen-2018]

This has been fixed by #15215