Make breadcrumb animation run on the compositing layer

res/css/structures/_RoomView.scss

@@ -237,6 +237,7 @@ hr.mx_RoomView_myReadMarker {
     position: relative;
     top: -1px;
     z-index: 1;
+    will-change: width;
     transition: width 400ms easeinsine 1s, opacity 400ms easeinsine 1s;
     width: 99%;
     opacity: 1;

res/css/views/rooms/_RoomBreadcrumbs.scss

@@ -32,14 +32,14 @@ limitations under the License.
     // first triggering the enter state with the newest breadcrumb off screen (-40px) then
     // sliding it into view.
     &.mx_RoomBreadcrumbs-enter {
-        margin-left: -40px; // 32px for the avatar, 8px for the margin
+        transform: translateX(-40px); // 32px for the avatar, 8px for the margin
     }
     &.mx_RoomBreadcrumbs-enter-active {
-        margin-left: 0;
+        transform: translateX(0);
 
         // Timing function is as-requested by design.
         // NOTE: The transition time MUST match the value passed to CSSTransition!
-        transition: margin-left 640ms cubic-bezier(0.66, 0.02, 0.36, 1);
+        transition: transform 640ms cubic-bezier(0.66, 0.02, 0.36, 1);
     }
 
     .mx_RoomBreadcrumbs_placeholder {

src/components/structures/ContextMenu.tsx

@@ -390,7 +390,7 @@ export class ContextMenu extends React.PureComponent<IProps, IState> {
     }
 
     render(): React.ReactChild {
-        return ReactDOM.createPortal(this.renderMenu(), document.body);
+        return ReactDOM.createPortal(this.renderMenu(), getOrCreateContainer());
     }
 }