Revert "remove fix for old chrome bug"

This reverts commit ecb074862e.

src/components/structures/ScrollPanel.js

@@ -215,6 +215,24 @@ module.exports = React.createClass({
         debuglog("Scroll event: offset now:", sn.scrollTop,
                  "_lastSetScroll:", this._lastSetScroll);
 
+        // Sometimes we see attempts to write to scrollTop essentially being
+        // ignored. (Or rather, it is successfully written, but on the next
+        // scroll event, it's been reset again).
+        //
+        // This was observed on Chrome 47, when scrolling using the trackpad in OS
+        // X Yosemite.  Can't reproduce on El Capitan. Our theory is that this is
+        // due to Chrome not being able to cope with the scroll offset being reset
+        // while a two-finger drag is in progress.
+        //
+        // By way of a workaround, we detect this situation and just keep
+        // resetting scrollTop until we see the scroll node have the right
+        // value.
+        if (this._lastSetScroll !== undefined && sn.scrollTop < this._lastSetScroll-200) {
+            console.log("Working around vector-im/vector-web#528");
+            this._restoreSavedScrollState();
+            return;
+        }
+
         // If there weren't enough children to fill the viewport, the scroll we
         // got might be different to the scroll we wanted; we don't want to
         // forget what we wanted, so don't overwrite the saved state unless