[jcheng5]

with @shelbylevel, and Posit Asisstant/Kimi K3

Fixes #1356

Wrap the top navigation bar in a <header> element so the page has a banner landmark, and render the content area as <main> when no sidebar is present (sidebar pages already get one via page_main_container()).

Both changes are made in page_navbar() rather than the shared navs_bar_() so that embedded navset_bar() instances don't produce inappropriate page-level landmarks. The <main> change is done in-place (the content div's tag is renamed) to preserve fill layout behavior.

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[gadenbuie]

Thanks for picking this up! Adding banner and main landmarks to page_navbar() is a real accessibility improvement, and keeping the change out of navset_bar() is the right call.

The blocking issue is that bslib's CSS for navbars assumes the .navbar and its content container are adjacent siblings, which is the shape navbarPage_() produces for page_navbar(), shiny::navbarPage(), and navset_bar() alike. Wrapping the nav in <header> and renaming the content div to <main> changes that shape for page_navbar() only, which causes visible layout regressions in existing apps (details inline). I tested an alternative that adds the same landmarks by renaming the existing elements instead, which keeps the DOM shape and needs no CSS changes.