Selector: Leverage the :scope pseudo-class where possible

The `:scope` pseudo-class[1] has surprisingly good browser support: Chrome,
Firefox & Safari have supported if for a long time; only IE & Edge lack support.
This commit leverages this pseudo-class to get rid of the ID hack in most cases.
Adding a temporary ID may cause layout thrashing which was reported a few times
in [the past.

We can't completely eliminate the ID hack in modern browses as sibling selectors
require us to change context to the parent and then `:scope` stops applying to
what we'd like. But it'd still improve performance in the vast majority of
cases.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:scope

Fixes jquery/jquery#4453
Ref jquery/jquery#4454
Ref jquery/jquery#4332
Ref jquerygh-405

src/sizzle.js

@@ -329,24 +329,30 @@ function Sizzle( selector, context, results, seed ) {
 				// Thanks to Andrew Dupont for this technique.
 				if ( nodeType === 1 && rdescend.test( selector ) ) {
 
-					// Capture the context ID, setting it first if necessary
-					if ( ( nid = context.getAttribute( "id" ) ) ) {
-						nid = nid.replace( rcssescape, fcssescape );
-					} else {
-						context.setAttribute( "id", ( nid = expando ) );
+					// Expand context for sibling selectors
+					newContext = rsibling.test( selector ) && testContext( context.parentNode ) ||
+						context;
+
+					// We can use :scope instead of the ID hack if the browser
+					// supports it & if we're not changing the context.
+					if ( newContext !== context || !support.scope ) {
+
+						// Capture the context ID, setting it first if necessary
+						if ( ( nid = context.getAttribute( "id" ) ) ) {
+							nid = nid.replace( rcssescape, fcssescape );
+						} else {
+							context.setAttribute( "id", ( nid = expando ) );
+						}
 					}
 
 					// Prefix every selector in the list
 					groups = tokenize( selector );
 					i = groups.length;
 					while ( i-- ) {
-						groups[ i ] = "#" + nid + " " + toSelector( groups[ i ] );
+						groups[ i ] = ( nid ? "#" + nid : ":scope" ) + " " +
+							toSelector( groups[ i ] );
 					}
 					newSelector = groups.join( "," );
-
-					// Expand context for sibling selectors
-					newContext = rsibling.test( selector ) && testContext( context.parentNode ) ||
-						context;
 				}
 
 				try {
@@ -626,6 +632,16 @@ setDocument = Sizzle.setDocument = function( node ) {
 		}
 	}
 
+	// Support: IE 9 - 11+, Edge 12 - 18+
+	// IE/Edge don't support the :scope pseudo-class.
+	// Support: Safari 6.0 only
+	// Safari 6.0 supports :scope but it's an alias of :root there.
+	support.scope = assert( function( el ) {
+		docElem.appendChild( el ).appendChild( document.createElement( "div" ) );
+		return typeof el.querySelectorAll !== "undefined" &&
+			!el.querySelectorAll( ":scope fieldset div" ).length;
+	} );
+
 	/* Attributes
 	---------------------------------------------------------------------- */
 

test/data/fixtures.html

@@ -269,5 +269,6 @@
 		</em>
 		<span id="siblingspan"></span>
 	</div>​
+
 	<br id="last"/>
 </div>

test/unit/selector.js

@@ -1367,6 +1367,46 @@ QUnit.test( "context", function( assert ) {
 	);
 } );
 
+( function() {
+    var scopeSupport;
+	try {
+		document.querySelectorAll( ":scope" );
+		scopeSupport = true;
+	} catch ( e ) {
+		scopeSupport = false;
+	}
+
+	// Support: IE 6 - 11+, Edge 12 - 18+, Chrome <=25 only, Safari <=6 only, Firefox <=13 only, Opera <=12 only
+	// Older browsers don't support the :scope pseudo-class so they may trigger MutationObservers.
+	// The test is skipped there.
+	QUnit[ scopeSupport && window.MutationObserver ? "test" : "skip" ](
+		"selectors maintaining context don't trigger mutation observers", function( assert ) {
+		assert.expect( 1 );
+
+		var timeout,
+			done = assert.async(),
+			elem = document.createElement( "div" );
+
+		elem.innerHTML = "<div></div>";
+
+		var observer = new MutationObserver(  function() {
+			clearTimeout( timeout );
+			observer.disconnect();
+			assert.ok( false, "Mutation observer fired during selection" );
+			done();
+		} );
+		observer.observe( elem, { attributes: true } );
+
+		Sizzle( "div div", elem );
+
+		timeout = setTimeout( function() {
+			observer.disconnect();
+			assert.ok( true, "Mutation observer didn't fire during selection" );
+			done();
+		} );
+	} );
+} )();
+
 QUnit.test( "caching", function( assert ) {
 	assert.expect( 3 );