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

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

src/selector.js

@@ -7,7 +7,8 @@ define( [
 	"./var/push",
 	"./selector/support",
 
-	"./selector/contains", // jQuery.contains
+	// The following utils are attached directly to the jQuery object.
+	"./selector/contains",
 	"./selector/escapeSelector"
 ], function( jQuery, document, indexOf, hasOwn, pop, push, support ) {
 
@@ -245,24 +246,30 @@ function find( 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 = jQuery.escapeSelector( nid );
-					} 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 = jQuery.escapeSelector( nid );
+						} 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 {

src/selector/support.js

@@ -49,6 +49,13 @@ support.disconnectedMatch = assert( function( el ) {
 	return matches.call( el, "*" );
 } );
 
+// Support: IE 9 - 11+, Edge 12 - 18+
+// IE/Edge don't support the :scope pseudo-class.
+try {
+	document.querySelectorAll( ":scope" );
+	support.scope = true;
+} catch ( e ) {}
+
 return support;
 
 } );

test/unit/selector.js

@@ -1640,6 +1640,41 @@ QUnit.test( "context", function( assert ) {
 	}
 } );
 
+// Support: IE 11+, Edge 12 - 18+
+// IE/Edge don't support the :scope pseudo-class so they will trigger MutationObservers.
+// The test is skipped there.
+QUnit[
+	( QUnit.isIE || /edge\//i.test( navigator.userAgent ) ) ?
+		"skip" :
+		"test"
+	]( "selectors maintaining context don't trigger mutation observers", function( assert ) {
+	assert.expect( 1 );
+
+	var timeout,
+		done = assert.async(),
+		container = jQuery( "<div/>" ),
+		child = jQuery( "<div/>" );
+
+	child.appendTo( container );
+	container.appendTo( "#qunit-fixture" );
+
+	var observer = new MutationObserver(  function() {
+		clearTimeout( timeout );
+		observer.disconnect();
+		assert.ok( false, "Mutation observer fired during selection" );
+		done();
+	} );
+	observer.observe( container[ 0 ], { attributes: true } );
+
+	container.find( "div div" );
+
+	timeout = setTimeout( function() {
+		observer.disconnect();
+		assert.ok( true, "Mutation observer didn't fire during selection" );
+		done();
+	} );
+} );
+
 QUnit.test( "caching does not introduce bugs", function( assert ) {
 	assert.expect( 3 );
 

test/unit/support.js

@@ -77,6 +77,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: false,
+				scope: false,
 				scrollboxSize: true
 			},
 			ie_10_11: {
@@ -98,6 +99,7 @@ testIframe(
 				radioValue: false,
 				reliableMarginLeft: true,
 				reliableTrDimensions: false,
+				scope: false,
 				scrollboxSize: true
 			},
 			ie_9: {
@@ -119,6 +121,7 @@ testIframe(
 				radioValue: false,
 				reliableMarginLeft: true,
 				reliableTrDimensions: false,
+				scope: false,
 				scrollboxSize: false
 			},
 			chrome: {
@@ -140,6 +143,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			safari: {
@@ -161,6 +165,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			safari_9_10: {
@@ -182,6 +187,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			firefox: {
@@ -203,6 +209,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: false,
+				scope: true,
 				scrollboxSize: true
 			},
 			firefox_60: {
@@ -224,6 +231,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: false,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			ios: {
@@ -245,6 +253,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			ios_9_10: {
@@ -266,6 +275,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			ios_8: {
@@ -287,6 +297,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			ios_7: {
@@ -308,6 +319,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: true,
 				reliableTrDimensions: true,
+				scope: true,
 				scrollboxSize: true
 			},
 			android: {
@@ -329,6 +341,7 @@ testIframe(
 				radioValue: true,
 				reliableMarginLeft: false,
 				reliableTrDimensions: true,
+				scope: false,
 				scrollboxSize: true
 			}
 		};
@@ -385,6 +398,15 @@ testIframe(
 			j++;
 		}
 
+		// Add an assertion per undefined support prop as it may
+		// not even exist on computedSupport but we still want to run
+		// the check.
+		for ( prop in expected ) {
+			if ( expected[ prop ] === undefined ) {
+				j++;
+			}
+		}
+
 		assert.expect( j );
 
 		for ( i in expected ) {
@@ -413,14 +435,23 @@ testIframe(
 			i++;
 		}
 
+		// Add an assertion per undefined support prop as it may
+		// not even exist on computedSupport but we still want to run
+		// the check.
+		for ( prop in expected ) {
+			if ( expected[ prop ] === undefined ) {
+				i++;
+			}
+		}
+
 		assert.expect( i );
 
 		// Record all support props and the failing ones and ensure everyone
 		// except a few on a whitelist are failing at least once.
 		for ( browserKey in expectedMap ) {
 			for ( supportTestName in expectedMap[ browserKey ] ) {
 				supportProps[ supportTestName ] = true;
-				if ( expectedMap[ browserKey ][ supportTestName ] !== true ) {
+				if ( !expectedMap[ browserKey ][ supportTestName ] ) {
 					failingSupportProps[ supportTestName ] = true;
 				}
 			}