I just upgraded to latest (2.8.0.beta4) after many months of negilgence and it seems that my fix wasn’t so sustainable (or perhaps the current issue is unrelated to this fix). I’m currently working to figure out what happened and will update here once I know, but if the following strikes a bell for anyone, please let me know.
So basically what happened is the same as described in the OP, except that this time it is not the discourse header covering the top navigation bar but the top navigation bar covering the discourse header . (edit: I now believe that the header is not covered but not rendered at all). When I disable the default theme via safe mode, it displays fine.
Strangely, though, when I navigate to /admin/customize/themes/11/desktop/scss/edit both the discourse header and the top navigation bar display correctly (both in safe mode and normal mode). I believe the reason why there is no difference between safe and normal mode on the theme page is that safe mode doesn’t apply to some of the admin pages. But what I don’t understand is why everything is displayed correctly on the admin pages but not otherwise.
Another thing I don’t understand: when I access the site on my mobile, it also displays correctly, even when I explicitly request the desktop version…
I will investigate further but would appreciate any hints here.
Edit: Here is what I see when I inspect a public page (in normal mode):
And here is what I see when I inspect an admin page (in normal mode):
As you can see, my CSS fix is present in both cases, but only the second one renders the page correctly…