Update the CSS transition for breadcrumbs

The actual transition length might need adjusting, but this is fairly close to what was requested.

package.json

@@ -94,6 +94,7 @@
     "react-dom": "^16.9.0",
     "react-focus-lock": "^2.2.1",
     "react-resizable": "^1.10.1",
+    "react-transition-group": "^4.4.1",
     "resize-observer-polyfill": "^1.5.0",
     "sanitize-html": "^1.18.4",
     "text-encoding-utf-8": "^1.0.1",
@@ -126,6 +127,7 @@
     "@types/qrcode": "^1.3.4",
     "@types/react": "^16.9",
     "@types/react-dom": "^16.9.8",
+    "@types/react-transition-group": "^4.4.0",
     "@types/zxcvbn": "^4.4.0",
     "babel-eslint": "^10.0.3",
     "babel-jest": "^24.9.0",

res/css/views/rooms/_RoomBreadcrumbs2.scss

@@ -14,34 +14,32 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-@keyframes breadcrumb-popin {
-    0% {
-        // Ideally we'd use `width` instead of `opacity`, but we only
-        // have 16 nanoseconds to render the frame, and width is expensive.
-        opacity: 0;
-        transform: scale(0);
-    }
-    100% {
-        opacity: 1;
-        transform: scale(1);
-    }
-}
-
 .mx_RoomBreadcrumbs2 {
+    width: 100%;
+
     // Create a flexbox for the crumbs
     display: flex;
     flex-direction: row;
     align-items: flex-start;
-    width: 100%;
 
     .mx_RoomBreadcrumbs2_crumb {
         margin-right: 8px;
         width: 32px;
-
-        // React loves to add elements, so only target the one we want to animate
-        &:first-child {
-            animation: breadcrumb-popin 0.3s;
     }
+
+    // These classes come from the CSSTransition component. There's many more classes we
+    // could care about, but this is all we worried about for now. The animation works by
+    // first triggering the enter state with the newest breadcrumb off screen (-40px) then
+    // sliding it into view.
+    &.mx_RoomBreadcrumbs2-enter {
+        margin-left: -40px; // 32px for the avatar, 8px for the margin
+    }
+    &.mx_RoomBreadcrumbs2-enter-active {
+        margin-left: 0;
+
+        // Timing function is as-requested by design.
+        // NOTE: The transition time MUST match the value passed to CSSTransition!
+        transition: margin-left 300ms cubic-bezier(0.66, 0.02, 0.36, 1);
     }
 
     .mx_RoomBreadcrumbs2_placeholder {

src/components/views/rooms/RoomBreadcrumbs2.tsx

@@ -23,6 +23,7 @@ import { Room } from "matrix-js-sdk/src/models/room";
 import defaultDispatcher from "../../../dispatcher/dispatcher";
 import Analytics from "../../../Analytics";
 import { UPDATE_EVENT } from "../../../stores/AsyncStore";
+import { CSSTransition, TransitionGroup } from "react-transition-group";
 
 /*******************************************************************
  *   CAUTION                                                       *
@@ -36,6 +37,14 @@ interface IProps {
 }
 
 interface IState {
+    // Both of these control the animation for the breadcrumbs. For details on the
+    // actual animation, see the CSS.
+    //
+    // doAnimation is to lie to the CSSTransition component (see onBreadcrumbsUpdate
+    // for info). skipFirst is used to try and reduce jerky animation - also see the
+    // breadcrumb update function for info on that.
+    doAnimation: boolean;
+    skipFirst: boolean;
 }
 
 export default class RoomBreadcrumbs2 extends React.PureComponent<IProps, IState> {
@@ -44,6 +53,11 @@ export default class RoomBreadcrumbs2 extends React.PureComponent<IProps, IState
     constructor(props: IProps) {
         super(props);
 
+        this.state = {
+            doAnimation: true, // technically we want animation on mount, but it won't be perfect
+            skipFirst: false, // render the thing, as boring as it is
+        };
+
         BreadcrumbsStore.instance.on(UPDATE_EVENT, this.onBreadcrumbsUpdate);
     }
 
@@ -54,7 +68,16 @@ export default class RoomBreadcrumbs2 extends React.PureComponent<IProps, IState
 
     private onBreadcrumbsUpdate = () => {
         if (!this.isMounted) return;
-        this.forceUpdate(); // we have no state, so this is the best we can do
+
+        // We need to trick the CSSTransition component into updating, which means we need to
+        // tell it to not animate, then to animate a moment later. This causes two updates
+        // which means two renders. The skipFirst change is so that our don't-animate state
+        // doesn't show the breadcrumb we're about to reveal as it causes a visual jump/jerk.
+        // The second update, on the next available tick, causes the "enter" animation to start
+        // again and this time we want to show the newest breadcrumb because it'll be hidden
+        // off screen for the animation.
+        this.setState({doAnimation: false, skipFirst: true});
+        setTimeout(() => this.setState({doAnimation: true, skipFirst: false}), 0);
     };
 
     private viewRoom = (room: Room, index: number) => {
@@ -77,14 +100,26 @@ export default class RoomBreadcrumbs2 extends React.PureComponent<IProps, IState
             )
         });
 
-        if (tiles.length === 0) {
+        if (tiles.length > 0) {
-            tiles.push(
+            // NOTE: The CSSTransition timeout MUST match the timeout in our CSS!
+            return (
+                <CSSTransition
+                    appear={true} in={this.state.doAnimation} timeout={300}
+                    classNames='mx_RoomBreadcrumbs2'
+                >
+                    <div className='mx_RoomBreadcrumbs2'>
+                        {tiles.slice(this.state.skipFirst ? 1 : 0)}
+                    </div>
+                </CSSTransition>
+            );
+        } else {
+            return (
+                <div className='mx_RoomBreadcrumbs2'>
                     <div className="mx_RoomBreadcrumbs2_placeholder">
                         {_t("No recently visited rooms")}
                     </div>
+                </div>
             );
         }
-
-        return <div className='mx_RoomBreadcrumbs2'>{tiles}</div>;
     }
 }

tsconfig.json

@@ -15,7 +15,8 @@
     "types": [
       "node",
       "react",
-      "flux"
+      "flux",
+      "react-transition-group"
     ]
   },
   "include": [

yarn.lock

@@ -968,7 +968,7 @@
     core-js-pure "^3.0.0"
     regenerator-runtime "^0.13.4"
 
-"@babel/runtime@^7.0.0", "@babel/runtime@^7.1.2", "@babel/runtime@^7.8.3", "@babel/runtime@^7.8.4":
+"@babel/runtime@^7.0.0", "@babel/runtime@^7.1.2", "@babel/runtime@^7.5.5", "@babel/runtime@^7.8.3", "@babel/runtime@^7.8.4", "@babel/runtime@^7.8.7":
   version "7.10.2"
   resolved "https://registry.yarnpkg.com/@babel/runtime/-/runtime-7.10.2.tgz#d103f21f2602497d38348a32e008637d506db839"
   integrity sha512-6sF3uQw2ivImfVIl62RZ7MXhO2tap69WeWK57vAaimT6AZbE4FbqjdEJIN1UqoD6wI6B+1n9UiagafH1sxjOtg==
@@ -1352,6 +1352,13 @@
   dependencies:
     "@types/react" "*"
 
+"@types/react-transition-group@^4.4.0":
+  version "4.4.0"
+  resolved "https://registry.yarnpkg.com/@types/react-transition-group/-/react-transition-group-4.4.0.tgz#882839db465df1320e4753e6e9f70ca7e9b4d46d"
+  integrity sha512-/QfLHGpu+2fQOqQaXh8MG9q03bFENooTb/it4jr5kKaZlDQfWvjqWZg48AwzPVMBHlRuTRAY7hRHCEOXz5kV6w==
+  dependencies:
+    "@types/react" "*"
+
 "@types/react@*", "@types/react@^16.9":
   version "16.9.35"
   resolved "https://registry.yarnpkg.com/@types/react/-/react-16.9.35.tgz#a0830d172e8aadd9bd41709ba2281a3124bbd368"
@@ -2835,7 +2842,7 @@ cssstyle@^1.0.0:
   dependencies:
     cssom "0.3.x"
 
-csstype@^2.2.0:
+csstype@^2.2.0, csstype@^2.6.7:
   version "2.6.10"
   resolved "https://registry.yarnpkg.com/csstype/-/csstype-2.6.10.tgz#e63af50e66d7c266edb6b32909cfd0aabe03928b"
   integrity sha512-D34BqZU4cIlMCY93rZHbrq9pjTAQJ3U8S8rfBqjwHxkGPThWFjzZDQpgMJY0QViLxth6ZKYiwFBo14RdN44U/w==
@@ -3054,6 +3061,14 @@ doctrine@^3.0.0:
   dependencies:
     esutils "^2.0.2"
 
+dom-helpers@^5.0.1:
+  version "5.1.4"
+  resolved "https://registry.yarnpkg.com/dom-helpers/-/dom-helpers-5.1.4.tgz#4609680ab5c79a45f2531441f1949b79d6587f4b"
+  integrity sha512-TjMyeVUvNEnOnhzs6uAn9Ya47GmMo3qq7m+Lr/3ON0Rs5kHvb8I+SQYjLUSYn7qhEm0QjW0yrBkvz9yOrwwz1A==
+  dependencies:
+    "@babel/runtime" "^7.8.7"
+    csstype "^2.6.7"
+
 dom-serializer@0, dom-serializer@^0.2.1:
   version "0.2.2"
   resolved "https://registry.yarnpkg.com/dom-serializer/-/dom-serializer-0.2.2.tgz#1afb81f533717175d478655debc5e332d9f9bb51"
@@ -7136,6 +7151,16 @@ react-test-renderer@^16.0.0-0, react-test-renderer@^16.9.0:
     react-is "^16.8.6"
     scheduler "^0.19.1"
 
+react-transition-group@^4.4.1:
+  version "4.4.1"
+  resolved "https://registry.yarnpkg.com/react-transition-group/-/react-transition-group-4.4.1.tgz#63868f9325a38ea5ee9535d828327f85773345c9"
+  integrity sha512-Djqr7OQ2aPUiYurhPalTrVy9ddmFCCzwhqQmtN+J3+3DzLO209Fdr70QrN8Z3DsglWql6iY1lDWAfpFiBtuKGw==
+  dependencies:
+    "@babel/runtime" "^7.5.5"
+    dom-helpers "^5.0.1"
+    loose-envify "^1.4.0"
+    prop-types "^15.6.2"
+
 react@^16.9.0:
   version "16.13.1"
   resolved "https://registry.yarnpkg.com/react/-/react-16.13.1.tgz#2e818822f1a9743122c063d6410d85c1e3afe48e"