Selector: Leverage the :scope pseudo-class where possible (no EdgeHTML support)

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

Author: mgol

src/selector.js

@@ -6,13 +6,13 @@ define( [
 	"./var/pop",
 	"./var/push",
 	"./selector/rbuggyQSA",
-	"./selector/support",
+	"./var/isIE",
 
 	// The following utils are attached directly to the jQuery object.
 	"./selector/contains",
 	"./selector/escapeSelector",
 	"./selector/uniqueSort"
-], function( jQuery, document, documentElement, indexOf, pop, push, rbuggyQSA, support ) {
+], function( jQuery, document, documentElement, indexOf, pop, push, rbuggyQSA, isIE ) {
 
 "use strict";
 
@@ -155,7 +155,7 @@ function selectorError( msg ) {
 }
 
 function find( selector, context, results, seed ) {
-	var m, i, elem, nid, match, groups, newSelector,
+	var m, i, elem, nid, match, groups, newSelector, canUseScope,
 		newContext = context && context.ownerDocument,
 
 		// nodeType defaults to 9, since context defaults to document
@@ -236,9 +236,10 @@ function find( selector, context, results, seed ) {
 
 					// 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 ) {
+					canUseScope = newContext === context && !isIE;
 
-						// Capture the context ID, setting it first if necessary
+					// Capture the context ID, setting it first if necessary
+					if ( !canUseScope ) {
 						if ( ( nid = context.getAttribute( "id" ) ) ) {
 							nid = jQuery.escapeSelector( nid );
 						} else {
@@ -250,7 +251,7 @@ function find( selector, context, results, seed ) {
 					groups = tokenize( selector );
 					i = groups.length;
 					while ( i-- ) {
-						groups[ i ] = ( nid ? "#" + nid : ":scope" ) + " " +
+						groups[ i ] = ( canUseScope || "#" + nid ) + " " +
 							toSelector( groups[ i ] );
 					}
 					newSelector = groups.join( "," );

test/unit/support.js

@@ -58,24 +58,12 @@ testIframe(
 	var expected,
 		userAgent = window.navigator.userAgent,
 		expectedMap = {
-			edge: {
-				scope: undefined
-			},
-			ie_11: {
-				scope: undefined
-			},
-			chrome: {
-				scope: true
-			},
-			safari: {
-				scope: true
-			},
-			firefox: {
-				scope: true
-			},
-			ios: {
-				scope: true
-			}
+			edge: {},
+			ie_11: {},
+			chrome: {},
+			safari: {},
+			firefox: {},
+			ios: {}
 		};
 
 	if ( /edge\//i.test( userAgent ) ) {