The fixed width and text wrapping cause this header to wrap.
https://contrib.drupalarchitect.info/project/mantra-theme/task/216-media-library-styles
Analysis
The Content w/ sidebar component, used in the layout currently uses --sidebar-second-width, which is set to 20rem by default, but can be overridden with UI Skins.
The contrib site had it set to 12rem. Bumping to 18rem was enough to fix it.
This needs some work. The sidebar first/second variables should be for the global regions. We need separate ones for content sidebars.
Alternatively, header text sizes could be reduced for sidebars.
Documentation
Issue status