[fix] UIManager measurements don't block

A large number of layout measurements (and their corresponding tasks)
can block the main thread. Make the work async and try to keep the UI
responsive to user input while the layout work is taking place.

packages/react-native-web/src/exports/UIManager/index.js

@@ -27,11 +27,13 @@ const getRect = node => {
 const measureLayout = (node, relativeToNativeNode, callback) => {
   const relativeNode = relativeToNativeNode || (node && node.parentNode);
   if (node && relativeNode) {
-    const relativeRect = getRect(relativeNode);
-    const { height, left, top, width } = getRect(node);
-    const x = left - relativeRect.left;
-    const y = top - relativeRect.top;
-    callback(x, y, width, height, left, top);
+    setTimeout(() => {
+      const relativeRect = getRect(relativeNode);
+      const { height, left, top, width } = getRect(node);
+      const x = left - relativeRect.left;
+      const y = top - relativeRect.top;
+      callback(x, y, width, height, left, top);
+    }, 0);
   }
 };
 
@@ -54,8 +56,10 @@ const UIManager = {
 
   measureInWindow(node, callback) {
     if (node) {
-      const { height, left, top, width } = getRect(node);
-      callback(left, top, width, height);
+      setTimeout(() => {
+        const { height, left, top, width } = getRect(node);
+        callback(left, top, width, height);
+      }, 0);
     }
   },
 

packages/react-native-web/src/modules/applyLayout/index.js

@@ -56,9 +56,7 @@ const observe = instance => {
     resizeObserver.observe(node);
   } else {
     instance._layoutId = id;
-    setTimeout(() => {
-      instance._handleLayout();
-    }, 0);
+    instance._handleLayout();
   }
 };
 
@@ -122,22 +120,24 @@ const applyLayout = Component => {
     const layout = this._layoutState;
     const { onLayout } = this.props;
 
-    if (this._isMounted && onLayout) {
+    if (onLayout) {
       this.measure((x, y, width, height) => {
-        if (
-          layout.x !== x ||
-          layout.y !== y ||
-          layout.width !== width ||
-          layout.height !== height
-        ) {
-          this._layoutState = { x, y, width, height };
-          const nativeEvent = {
-            layout: this._layoutState,
-            get target() {
-              return findNodeHandle(this);
-            }
-          };
-          onLayout({ nativeEvent, timeStamp: Date.now() });
+        if (this._isMounted) {
+          if (
+            layout.x !== x ||
+            layout.y !== y ||
+            layout.width !== width ||
+            layout.height !== height
+          ) {
+            this._layoutState = { x, y, width, height };
+            const nativeEvent = {
+              layout: this._layoutState,
+              get target() {
+                return findNodeHandle(this);
+              }
+            };
+            onLayout({ nativeEvent, timeStamp: Date.now() });
+          }
         }
       });
     }