Fix bug which stopped us scrolling down after we scrolled up

Make sure that, if we scroll up enough to move the timelinewindow away from the
end of the timeline, we reset the canForwardPaginate flag.

test/test-utils.js

@@ -42,8 +42,8 @@ module.exports.stubClient = function() {
         sendReadReceipt: sinon.stub().returns(q()),
     };
 
-    // create the peg
-
+    // stub out the methods in MatrixClientPeg
+    //
     // 'sandbox.restore()' doesn't work correctly on inherited methods,
     // so we do this for each method
     var methods = ['get', 'unset', 'replaceUsingUrls',
@@ -51,7 +51,9 @@ module.exports.stubClient = function() {
     for (var i = 0; i < methods.length; i++) {
         sandbox.stub(peg, methods[i]);
     }
-    peg.get.returns(client);
+    // MatrixClientPeg.get() is called a /lot/, so implement it with our own
+    // fast stub function rather than a sinon stub
+    peg.get = function() { return client; };
     return sandbox;
 }