Don't paginate whilst decrypting events

As comment hopefully explains. This meant loading the app on an e2e
room would often be very slow as it tried to pull hundreds of events
into the timeline.

src/components/structures/TimelinePanel.js

@@ -302,6 +302,8 @@ var TimelinePanel = React.createClass({
 
     // set off a pagination request.
     onMessageListFillRequest: function(backwards) {
+        if (!this._shouldPaginate()) return Promise.resolve(false);
+
         const dir = backwards ? EventTimeline.BACKWARDS : EventTimeline.FORWARDS;
         const canPaginateKey = backwards ? 'canBackPaginate' : 'canForwardPaginate';
         const paginatingKey = backwards ? 'backPaginating' : 'forwardPaginating';
@@ -1091,6 +1093,17 @@ var TimelinePanel = React.createClass({
         }, this.props.onReadMarkerUpdated);
     },
 
+    _shouldPaginate: function() {
+        // don't try to paginate while events in the timeline are
+        // still being decrypted. We don't render events while they're
+        // being decrypted, so they don't take up space in the timeline.
+        // This means we can pull quite a lot of events into the timeline
+        // and end up trying to render a lot of events.
+        return !this.state.events.some((e) => {
+            return e.isBeingDecrypted();
+        });
+    },
+
     render: function() {
         const MessagePanel = sdk.getComponent("structures.MessagePanel");
         const Loader = sdk.getComponent("elements.Spinner");