Closes #21055: Prevents ScrollSpy from clearing active item when Safari rubberbands (#21056)

When the rubberband effect causes Safari to scroll past the top of the
page, the value of scrollTop becomes negative. If the offset of the first
ScrollSpy target is 0 - essentially if the target is at the top of the
page - then ScrollSpy should not clear the active item. Conceptually, the
first item should remain active when rubberbanding past the top of the
page.

This commit fixes issue #21055 by verifying the first scrollspy target is
not at the top of the page before clearing the active nav-item.

js/src/scrollspy.js

@@ -221,7 +221,7 @@ const ScrollSpy = (($) => {
         return
       }
 
-      if (this._activeTarget && scrollTop < this._offsets[0]) {
+      if (this._activeTarget && scrollTop < this._offsets[0] && this._offsets[0] > 0) {
         this._activeTarget = null
         this._clear()
         return

js/tests/unit/scrollspy.js

@@ -287,6 +287,50 @@ $(function () {
       .scrollTop(201)
   })
 
+  QUnit.test('should NOT clear selection if above the first section and first section is at the top', function (assert) {
+    assert.expect(4)
+    var done = assert.async()
+
+    var sectionHTML = '<div id="header" style="height: 500px;"></div>'
+        + '<nav id="navigation" class="navbar">'
+        + '<ul class="nav navbar-nav">'
+        + '<li><a id="one-link"   class="nav-link active" href="#one">One</a></li>'
+        + '<li><a id="two-link"   class="nav-link" href="#two">Two</a></li>'
+        + '<li><a id="three-link" class="nav-link" href="#three">Three</a></li>'
+        + '</ul>'
+        + '</nav>'
+    $(sectionHTML).appendTo('#qunit-fixture')
+
+    var negativeHeight = -10
+    var startOfSectionTwo = 101
+
+    var scrollspyHTML = '<div id="content" style="height: 200px; overflow-y: auto;">'
+        + '<div id="one" style="height: 100px;"/>'
+        + '<div id="two" style="height: 100px;"/>'
+        + '<div id="three" style="height: 100px;"/>'
+        + '<div id="spacer" style="height: 100px;"/>'
+        + '</div>'
+    var $scrollspy = $(scrollspyHTML).appendTo('#qunit-fixture')
+
+    $scrollspy
+      .bootstrapScrollspy({
+        target: '#navigation',
+        offset: $scrollspy.position().top
+      })
+      .one('scroll', function () {
+        assert.strictEqual($('.active').length, 1, '"active" class on only one element present')
+        assert.strictEqual($('.active').is('#two-link'), true, '"active" class on second section')
+        $scrollspy
+          .one('scroll', function () {
+            assert.strictEqual($('.active').length, 1, '"active" class on only one element present')
+            assert.strictEqual($('.active').is('#one-link'), true, '"active" class on first section')
+            done()
+          })
+          .scrollTop(negativeHeight)
+      })
+      .scrollTop(startOfSectionTwo)
+  })
+
   QUnit.test('should correctly select navigation element on backward scrolling when each target section height is 100%', function (assert) {
     assert.expect(5)
     var navbarHtml =