Header messes up on mid-size viewports on the Horizon Theme

[nathank (Nathan Kershaw)]

I can reproduce it by simply narrowing the window of my browser on my desktop:

When sufficiently wide:

A bit narrower, and a few weird things happen (sidebar / header go white) which I’m not super sure is intentional:

Narrower highlights the problem (i.e. folding phone size)

Very narrow (normal phone size) is fine again.