Tests: fix supportsScroll feature test in offset

- iOS 12 and below do not support scrollTop/Left on the
  window or document. Before 82169df, window.supportsScroll
  was always undefined. There were 2 issues: (1) iOS <=12 do
  support scroll props on the body, so the support test
  wasn't helpful. (2) one test checked the wrong window and
  the value was always undefined, which meant those tests
  never ran in any browser.

Closes jquerygh-5506

Author: timmywil

test/unit/offset.js

@@ -4,7 +4,7 @@ if ( !includesModule( "offset" ) ) {
 	return;
 }
 
-var alwaysScrollable,
+var alwaysScrollable, supportsFixedPosition, supportsScroll,
 	forceScroll = supportjQuery( "<div></div>" ).css( { width: 2000, height: 2000 } ),
 	checkSupport = function( assert ) {
 
@@ -14,13 +14,14 @@ var alwaysScrollable,
 		var checkFixed = supportjQuery( "<div/>" )
 			.css( { position: "fixed", top: "20px" } )
 			.appendTo( "#qunit-fixture" );
-		window.supportsFixedPosition = checkFixed[ 0 ].offsetTop === 20;
+		supportsFixedPosition = checkFixed[ 0 ].offsetTop === 20;
 		checkFixed.remove();
 
 		// Append forceScroll to the body instead of #qunit-fixture because the latter is hidden
 		forceScroll.appendTo( "body" );
 		window.scrollTo( 200, 200 );
-		window.supportsScroll = document.documentElement.scrollTop || document.body.scrollTop;
+		supportsScroll = !!document.documentElement.scrollTop;
+
 		forceScroll.detach();
 
 		// Support: iOS <=7
@@ -369,7 +370,7 @@ testIframe( "static", "offset/static.html", function( assert, $ ) {
 	} );
 } );
 
-testIframe( "fixed", "offset/fixed.html", function( assert, $, window ) {
+testIframe( "fixed", "offset/fixed.html", function( assert, $ ) {
 	assert.expect( 38 );
 
 	var tests, $noTopLeft;
@@ -392,15 +393,15 @@ testIframe( "fixed", "offset/fixed.html", function( assert, $, window ) {
 	];
 
 	jQuery.each( tests, function() {
-		if ( !window.supportsScroll ) {
+		if ( !supportsScroll ) {
 			assert.ok( true, "Browser doesn't support scroll position." );
 			assert.ok( true, "Browser doesn't support scroll position." );
 			assert.ok( true, "Browser doesn't support scroll position." );
 			assert.ok( true, "Browser doesn't support scroll position." );
 			assert.ok( true, "Browser doesn't support scroll position." );
 			assert.ok( true, "Browser doesn't support scroll position." );
 
-		} else if ( window.supportsFixedPosition ) {
+		} else if ( supportsFixedPosition ) {
 			assert.equal( jQuery.isPlainObject( $( this.id ).offset() ), true, "jQuery('" + this.id + "').offset() is plain object" );
 			assert.equal( jQuery.isPlainObject( $( this.id ).position() ), true, "jQuery('" + this.id + "').position() is plain object" );
 			assert.equal( $( this.id ).offset().top,  this.offsetTop,  "jQuery('" + this.id + "').offset().top" );
@@ -429,7 +430,7 @@ testIframe( "fixed", "offset/fixed.html", function( assert, $, window ) {
 	];
 
 	jQuery.each( tests, function() {
-		if ( window.supportsFixedPosition ) {
+		if ( supportsFixedPosition ) {
 			$( this.id ).offset( { "top": this.top, "left": this.left } );
 			assert.equal( $( this.id ).offset().top,  this.top,  "jQuery('" + this.id + "').offset({ top: "  + this.top  + " })" );
 			assert.equal( $( this.id ).offset().left, this.left, "jQuery('" + this.id + "').offset({ left: " + this.left + " })" );
@@ -454,7 +455,7 @@ testIframe( "fixed", "offset/fixed.html", function( assert, $, window ) {
 
 	// Bug 8316
 	$noTopLeft = $( "#fixed-no-top-left" );
-	if ( window.supportsFixedPosition ) {
+	if ( supportsFixedPosition ) {
 		assert.equal( $noTopLeft.offset().top,  1007,  "Check offset top for fixed element with no top set" );
 		assert.equal( $noTopLeft.offset().left, 1007, "Check offset left for fixed element with no left set" );
 	} else {
@@ -503,7 +504,7 @@ testIframe( "scroll", "offset/scroll.html", function( assert, $, win ) {
 
 	win.name = "test";
 
-	if ( !window.supportsScroll ) {
+	if ( !supportsScroll ) {
 		assert.ok( true, "Browser doesn't support scroll position." );
 		assert.ok( true, "Browser doesn't support scroll position." );