Hide scrollbar if content height is smaller than scroll node height (#7220)

res/css/structures/_RoomView.scss

@@ -120,6 +120,10 @@ limitations under the License.
     overflow-y: auto;
     flex: 1 1 0;
     overflow-anchor: none;
+
+    &[data-scrollbar=false] {
+        overflow-y: hidden;
+    }
 }
 
 .mx_RoomView_messagePanelSearchSpinner {

src/components/structures/ScrollPanel.tsx

@@ -750,6 +750,8 @@ export default class ScrollPanel extends React.Component<IProps> {
         const minHeight = sn.clientHeight;
         const height = Math.max(minHeight, contentHeight);
         this.pages = Math.ceil(height / PAGE_SIZE);
+        const displayScrollbar = contentHeight > minHeight;
+        sn.dataset.scrollbar = displayScrollbar.toString();
         this.bottomGrowth = 0;
         const newHeight = `${this.getListHeight()}px`;