Background

As part of T313060 the footer was moved out of the grid container (.mw-page-container-inner). Given the footer and header both don't benefit from the grid, we should also move the header outside for consistency. This reduces the number of elements relying on grid for layout, and scopes our grid implementation to the main content.

This also has a few benefits to our CSS:

• Simplifies the header layout CSS, no longer have to style it differently based on the viewports and whether or not grid is enabled/disabled
• Allows us to move the main menu checkbox out of the grid container, allowing us to change the grid columns and rows based off the main menu state, rather than updating individual grid areas.

AC

• The header is outside of .mw-page-container-inner with no visual changes
• No visual changes for cached HTML
• Header and grid CSS is simplified

Developer notes

This patch needs to consider cached HTML. Fortunately for this change the original grid CSS should continue to work even when we move the header. See POC patch here