use box-sizing: border-box to make clientHeight === actual height

res/css/structures/_RoomView.scss

@@ -171,6 +171,10 @@ limitations under the License.
 .mx_RoomView_MessageList {
     list-style-type: none;
     padding: 18px;
+    margin: 0;
+    /* needed as min-height is set to clientHeight in ScrollPanel
+    to prevent shrinking when WhoIsTypingTile is hidden */
+    box-sizing: border-box;
 }
 
 .mx_RoomView_MessageList li {

src/components/structures/ScrollPanel.js

@@ -689,7 +689,7 @@ module.exports = React.createClass({
     blockShrinking: function() {
         const messageList = this.refs.itemlist;
         if (messageList) {
-            const currentHeight = messageList.clientHeight - 18;
+            const currentHeight = messageList.clientHeight;
             messageList.style.minHeight = `${currentHeight}px`;
         }
     },