The “Dropdown” mode and the admin sidebar feel quite incompatible at the moment. Here is what I am seeing locally:
Step 1: on homepage, I see no sidebar, I see the dropdown menu, as expected:
Step 2: click on Admin, the sidebar shows up:
Step 3: click on All Site Settings in the sidebar, sidebar is no longer there (but it is in the dropdown menu):
Same happens with all other links, except for those under Account (Backups, What’s New), which keep the sidebar in place.
Here’s a recording as well, you can see the jumpiness more clearly: