Create wrapper for all junk below firstHeading, including nav stuff

And put the position:relative on that. Keeps the sticky header out of
it, regardless, and let folks do their weird non-content voodoo on
stuff like the siteSub more consistently, I guess...

Bug: T231400
Change-Id: I7ac35f741210a0c54976f43d2294af8f123967a1

includes/TimelessTemplate.php

@@ -126,6 +126,7 @@ class TimelessTemplate extends BaseTemplate {
 				],
 				$this->get( 'title' )
 			) .
+			Html::rawElement( 'div', [ 'id' => 'bodyContentOuter' ],
 				Html::rawElement( 'div', [ 'id' => 'siteSub' ], $this->getMsg( 'tagline' )->parse() ) .
 				Html::rawElement( 'div', [ 'id' => 'mw-page-header-links' ],
 					$this->getPortlet(
@@ -154,6 +155,7 @@ class TimelessTemplate extends BaseTemplate {
 					$this->get( 'bodytext' ) .
 					$this->getClear()
 				)
+			)
 		);
 
 		return Html::rawElement( 'div', [ 'id' => 'mw-content' ], $html );

resources/screen-common.less

@@ -22,7 +22,7 @@ body {
 	padding: 1em @content-padding 3em;
 }
 
-#mw-content-text {
+#bodyContentOuter {
 	/* Set a z-index for the content to group it away from any nav stuff (extensions) - T200148 */
 	position: relative;
 	z-index: 0;