Stop table of contents sticky header overlapping focused items

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Remove aria-hidden from search label to let assistive technologies see its accessible name
 - Use hidden attribute to show/hide expiry notices instead of just CSS
 - Only use dialog role for table of contents when it behaves like one (accessibility fix)
+- Prevent interactive elements being obscured by sticky table of contents header
 
 ## 3.5.0
 

lib/assets/javascripts/_modules/table-of-contents.js

@@ -1,6 +1,39 @@
 (function ($, Modules) {
   'use strict'
 
+  // Most of the code below is gratefully taken from:
+  // https://www.tpgi.com/prevent-focused-elements-from-being-obscured-by-sticky-headers/
+  var StickyOverlapMonitors = function ($sticky) {
+    this.$sticky = $sticky
+    this.offset = 0
+    this.onFocus = this.showObscured.bind(this)
+    this.isMonitoring = false
+  }
+  StickyOverlapMonitors.prototype.run = function () {
+    var stickyIsVisible = this.$sticky.is(':visible')
+    if (stickyIsVisible && !this.isMonitoring) {
+      document.addEventListener('focus', this.onFocus, true)
+      this.isMonitoring = true
+    }
+    if (!stickyIsVisible && this.isMonitoring) {
+      document.removeEventListener('focus', this.onFocus, true)
+      this.isMonitoring = false
+    }
+  }
+  StickyOverlapMonitors.prototype.showObscured = function () {
+    var focused = document.activeElement || document.body
+    var applicable = focused !== document.body
+
+    if (!applicable) { return }
+
+    var stickyEdge = this.$sticky.get(0).getBoundingClientRect().bottom + this.offset
+    var diff = focused.getBoundingClientRect().top - stickyEdge
+
+    if (diff < 0) {
+      $(window).scrollTop($(window).scrollTop() + diff)
+    }
+  }
+
   Modules.TableOfContents = function () {
     var $html = $('html')
 
@@ -10,6 +43,8 @@
     var $openButton
     var $closeButton
 
+    var stickyOverlapMonitors
+
     this.start = function ($element) {
       $toc = $element
       $tocList = $toc.find('.js-toc-list')
@@ -19,6 +54,8 @@
 
       fixRubberBandingInIOS()
       updateAriaAttributes()
+      stickyOverlapMonitors = new StickyOverlapMonitors($('.fixedsticky'))
+      stickyOverlapMonitors.run()
 
       // Need delegated handler for show link as sticky polyfill recreates element
       $openButton.on('click.toc', preventingScrolling(openNavigation))
@@ -27,7 +64,10 @@
 
       // Allow aria hidden to be updated when resizing from mobile to desktop or
       // vice versa
-      $(window).on('resize.toc', updateAriaAttributes)
+      $(window).on('resize.toc', function () {
+        updateAriaAttributes()
+        stickyOverlapMonitors.run()
+      })
 
       $(document).on('keydown.toc', function (event) {
         var ESC_KEY = 27

package.json

@@ -23,6 +23,9 @@
       "it",
       "assert",
       "expect",
+      "Document",
+      "Element",
+      "Event",
       "GOVUK",
       "lunr",
       "$",

spec/javascripts/table-of-contents-spec.js

@@ -300,4 +300,138 @@ describe('Table of contents', function () {
       expect(scrollTopSpy).toHaveBeenCalledWith(399)
     })
   })
+
+  describe('Prevent table of contents open button overlapping focused elements', function () {
+    var _getBoundingClientRect
+    var _addEventListener
+    var _scrollTop
+    var $link
+    var $tocStickyHeader
+
+    beforeEach(function () {
+      _getBoundingClientRect = Element.prototype.getBoundingClientRect
+      _addEventListener = Document.prototype.addEventListener
+      _scrollTop = $.fn.scrollTop
+
+      $tocStickyHeader = $('.toc-show')
+      $link = $('<a href="">Test link</a>')
+      $('body').append($link)
+    })
+
+    afterEach(function () {
+      Element.prototype.getBoundingClientRect = _getBoundingClientRect
+      Document.prototype.addEventListener = _addEventListener
+      $.fn.extend({ scrollTop: _scrollTop })
+
+      $link.remove()
+    })
+
+    it('if an element is focused while being overlaped by the table of contents sticky header, the screen should scroll to reveal it', function () {
+      var tocStickyHeaderBottomPos = 50
+      var linkTopPos = 30
+      var windowScrollPos = 300
+      var scrollTopSpy = jasmine.createSpy('scrollTop')
+
+      $html.addClass('mobile-size') // the open button only appears on mobile-size screens
+
+      module = new GOVUK.Modules.TableOfContents()
+      module.start($toc)
+
+      // stub DOM APIs used to work out if an element is overlaped
+      Element.prototype.getBoundingClientRect = function () {
+        if (this === $tocStickyHeader.get(0)) {
+          return {
+            bottom: tocStickyHeaderBottomPos
+          }
+        }
+        if (this === $link.get(0)) {
+          return {
+            top: linkTopPos
+          }
+        }
+      }
+      $.fn.scrollTop = function (yPos) {
+        if (this.get(0) !== window) { return _scrollTop(arguments) }
+        if (yPos === undefined) { // call for current scrollTop position
+          return windowScrollPos
+        } else {
+          scrollTopSpy(yPos)
+        }
+      }
+
+      // replicating a real event requires us to focus the element and fire the event ourselves
+      $link.focus()
+      $link.get(0).dispatchEvent(new Event('focus'))
+
+      expect(scrollTopSpy).toHaveBeenCalledWith(windowScrollPos - (tocStickyHeaderBottomPos - linkTopPos))
+
+      $html.removeClass('mobile-size')
+    })
+
+    it('if the table of contents sticky header isn\'t shown, no focus tracking should happen', function () {
+      var scrollTopSpy = jasmine.createSpy('scrollTop')
+      var getBoundingClientRectSpy = jasmine.createSpy('getBoundingClientRectSpy')
+
+      module = new GOVUK.Modules.TableOfContents()
+      module.start($toc)
+
+      // stub out web APIs used if focus tracking runs
+      Element.prototype.getBoundingClientRect = function () {
+        if ((this === $tocStickyHeader.get(0)) || (this === $link.get(0))) {
+          getBoundingClientRectSpy()
+          return {
+            bottom: 50,
+            top: 30
+          }
+        }
+      }
+      $.fn.scrollTop = function (yPos) {
+        if (this.get(0) !== window) { return _scrollTop(arguments) }
+        scrollTopSpy(arguments)
+      }
+
+      // replicating a real event requires us to focus the element and fire the event ourselves
+      $link.focus()
+      $link.get(0).dispatchEvent(new Event('focus'))
+
+      expect(getBoundingClientRectSpy).not.toHaveBeenCalled()
+      expect(scrollTopSpy).not.toHaveBeenCalled()
+    })
+
+    it('if the table of contents sticky header shows but then is hidden when the screen resizes, no focus tracking should happen', function () {
+      var scrollTopSpy = jasmine.createSpy('scrollTop')
+      var getBoundingClientRectSpy = jasmine.createSpy('getBoundingClientRectSpy')
+
+      $html.addClass('mobile-size') // the open button only appears on mobile-size screens
+
+      module = new GOVUK.Modules.TableOfContents()
+      module.start($toc)
+
+      // simulate screen resizing to desktop-size
+      $html.removeClass('mobile-size')
+      $(window).trigger('resize')
+
+      // stub out web APIs used if focus tracking runs
+      Element.prototype.getBoundingClientRect = function () {
+        if ((this === $tocStickyHeader.get(0)) || (this === $link.get(0))) {
+          getBoundingClientRectSpy()
+          return {
+            bottom: 50,
+            top: 30
+          }
+        }
+      }
+      $.fn.scrollTop = function (yPos) {
+        if (this.get(0) !== window) { return _scrollTop(arguments) }
+        scrollTopSpy(arguments)
+      }
+
+      // replicating a real event requires us to focus the element and fire the event ourselves
+      $link.focus()
+      $link.get(0).dispatchEvent(new Event('focus'))
+
+      expect(getBoundingClientRectSpy).not.toHaveBeenCalled()
+      expect(scrollTopSpy).not.toHaveBeenCalled()
+    })
+  })
 })