Top menu does not appear on android in vertical mode

[Popolechien]

So if I open phzh_core-english-one_en_2020-09.zim on my phone (huawei android 9) the menu at the top does not appear and I'm stuck on a rather boring landing page

If I tilt the phone horizontally and the image resize then I can see the menu (if I tilt back it disappears again)

[rgaudin]

This apparently due to the size of the viewport. I can replicate that with a small device factor on a regular browser.

// assets/main.css @(max-width: 640px)
.zim-course-index, .zim-courseware {
    display: none;
}