Unstick performance bar
Fixes #118619
Moving to main content area gives us more area (fewer sticky headers!), and makes it much simpler when adding other sticky/fixed elements (no more .with-performance-bar
special handling for sidebars and things).
Wrapping content means it doesn't spill over the side on small screens. Also reduced line height a little so it doesn't take up too much space when wrapping.
Edited by Simon Knox