Fix a flaky test in the timelinepanel code

Sometimes it was possible for there to be a scroll event before the initial
pagination completed, which then upset the rest of the test. Just give it a few
ms to sort itself out instead.

src/components/structures/TimelinePanel.js

@@ -350,9 +350,9 @@ var TimelinePanel = React.createClass({
         });
     },
 
-    onMessageListScroll: function() {
+    onMessageListScroll: function(e) {
         if (this.props.onScroll) {
-            this.props.onScroll();
+            this.props.onScroll(e);
         }
 
         if (this.props.manageReadMarkers) {

test/components/structures/TimelinePanel-test.js

@@ -126,10 +126,15 @@ describe('TimelinePanel', function() {
             timeline.addEvent(mkMessage(i));
         }
 
-        var scrollDefer;
+        let scrollDefer;
+        const onScroll = (e) => {
+            console.log(`TimelinePanel called onScroll: ${e.target.scrollTop}`);
+            if (scrollDefer) {
+                scrollDefer.resolve();
+            }
+        };
         var rendered = ReactDOM.render(
-                <WrappedTimelinePanel timelineSet={timelineSet} onScroll={() => {scrollDefer.resolve()}}
-                />,
+                <WrappedTimelinePanel timelineSet={timelineSet} onScroll={onScroll} />,
                 parentDiv,
         );
         var panel = rendered.refs.panel;
@@ -152,9 +157,8 @@ describe('TimelinePanel', function() {
             return scrollDefer.promise;
         };
 
-        // wait for the panel to load - we'll get a scroll event once it
-        // happens
-        awaitScroll().then(() => {
+        // let the first round of pagination finish off
+        q.delay(5).then(() => {
             expect(panel.state.canBackPaginate).toBe(false);
             expect(scryEventTiles(panel).length).toEqual(N_EVENTS);
 
@@ -164,6 +168,8 @@ describe('TimelinePanel', function() {
 
             // wait for the scroll event to land
         }).then(awaitScroll).then(() => {
+            expect(scrollingDiv.scrollTop).toEqual(0);
+
             // there should be no pagination going on now
             expect(panel.state.backPaginating).toBe(false);
             expect(panel.state.forwardPaginating).toBe(false);