Steps to reproduce the issue

1. In backend on a desktop screen, reduce browser window width until the admin menu on the left hand side disappears and the burger button is shown at the bottom right corner.
2. Click on the burger button.
   Result: The admin menu which is now at the bottom of the screen is expanded => ok.
3. Click again on the burger button.
   Result: The admin menu at the bottom of the screen is collapsed => ok.
4. Enlarge browser window width until the burger button disappears.
   Result: See section "Actual result" below.

The issue happened to me also sometimes when skipping steps 2 and 3, but only when the particular page was shown for the very first time in that session, and even then not in all cases, so it was hard to reproduce. With steps 2 and 3 I could reproduce it reliably.

There are no Javascript errors shown in the broswer console.

Expected result

The admin menu appears again at the left hand side, preferrably with collapsed status.

Actual result

The admin menu remains invisible.

After a page reload the admin menu appears with collapsed status on the left hand side.

System information (as much as possible)

Clean installation of current 4.0-dev branch, recvent 4.0 nightly build or 4.0 Beta 1.

Additional comments

I've noticed this issue when testing PR #30131 . It happens with and without that PR applied. I think the problem is that in file build/media_source/templates/atum/js/template.es6.js, the setMobile functions sets some CSS class on either the wrapper or the sidebar-wrapper which are not properly reset by the setDesktop function. But I haven't found the error yet.

I have no idea if we have already an issue for that so this here is a duplicate. I haven't found one with a quick search for "burger" and a few other keywords, but I haven't checked deeper.