Decrypt last events first to avoid shifts when scrolling up

src/components/structures/TimelinePanel.js

@@ -1142,7 +1142,12 @@ class TimelinePanel extends React.Component {
     _getEvents() {
         const events = this._timelineWindow.getEvents();
 
+
+        // `slice` performs a shallow copy of the array
+        // we want the last event to be decrypted first but displayed last
+        // `reverse` is destructive and unfortunately mutates the "events" array
         events
+            .slice().reverse()
             .forEach(event => {
                 if (event.shouldAttemptDecryption()) {
                     event.attemptDecryption(MatrixClientPeg.get()._crypto);