fix: account for div container during infinite scroll

why:
* the top_navigation_placement feature flag introduces
a wrapper div around all of Canvas, to make the Drawer
implementation work
* InfiniteScroll looks at the document for scroll events,
but with the wrapper div there aren't any
* use the wrapper div if present but fall back to the usual
document-style implementation

closes INTEROP-8799
flag=none

test plan
* create more than 50 tools in a Course or Account
* easiest way to do that is to take an existing tool and in a rails console
with the tool as `t`, run this:
`100.times.each { |i| t.dup.tap{ |t| t.name = "test #{i}"; t.save }`
* with the top_navigation_placement flag off, scrolling down to
the bottom of the list of apps should load more
* with the flag enabled, scrolling down won't load any more
* with this commit checked out, scrolling down should
always load more tools

Change-Id: Ic65166fa597c296d4c010e0a87faf2992bea2b33
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/355123
Reviewed-by: Dustin Cowles <dustin.cowles@instructure.com>
Reviewed-by: Tucker Mcknight <tmcknight@instructure.com>
Reviewed-by: Ryan Hawkins <ryan.hawkins@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Ryan Hawkins <ryan.hawkins@instructure.com>
Product-Review: Xander Moffatt <xmoffatt@instructure.com>

ui/shared/infinite-scroll/react/components/InfiniteScroll.jsx

@@ -33,6 +33,9 @@ export default class InfiniteScroll extends React.Component {
 
   componentDidMount() {
     this.pageLoaded = this.props.pageStart
+    if (document.getElementById('drawer-layout-content')) {
+      this.scrollContainer = document.getElementById('drawer-layout-content')
+    }
     if (this.props.scrollContainer) {
       this.scrollContainer = this.props.scrollContainer
     }

ui/shared/infinite-scroll/react/components/__tests__/InfiniteScroll.test.jsx

@@ -123,5 +123,15 @@ describe('InfiniteScroll', () => {
 
       expect(windowSpy.mock.calls.map(c => c[0])).toContain('scroll')
     })
+
+    it('uses top nav drawer as container if present', () => {
+      const scrollContainer = document.createElement('div')
+      scrollContainer.id = 'drawer-layout-content'
+      document.body.append(scrollContainer)
+
+      const windowSpy = jest.spyOn(window, 'addEventListener')
+      render(<InfiniteScroll {...defaultProps()} />)
+      expect(windowSpy.mock.calls.map(c => c[0])).not.toContain('scroll')
+    })
   })
 })