From 5a38bac3fd48713fb660723a279fd3d21d081878 Mon Sep 17 00:00:00 2001
From: Dan Abramov <dan.abramov@me.com>
Date: Mon, 27 Jul 2020 16:03:44 +0100
Subject: [PATCH] Don't emulate bubbling of the scroll event

---
 .../__tests__/ReactDOMEventListener-test.js   | 110 +++++++++++++++---
 .../src/events/plugins/SimpleEventPlugin.js   |  16 ++-
 2 files changed, 107 insertions(+), 19 deletions(-)

diff --git a/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js b/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
index 86ddbbff30025..a1d8453e2b912 100644
--- a/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMEventListener-test.js
@@ -539,21 +539,15 @@ describe('ReactDOMEventListener', () => {
     const container = document.createElement('div');
     const ref = React.createRef();
     const onPlay = jest.fn();
-    const onScroll = jest.fn();
     const onCancel = jest.fn();
     const onClose = jest.fn();
     document.body.appendChild(container);
     try {
       ReactDOM.render(
-        <div
-          onPlay={onPlay}
-          onScroll={onScroll}
-          onCancel={onCancel}
-          onClose={onClose}>
+        <div onPlay={onPlay} onCancel={onCancel} onClose={onClose}>
           <div
             ref={ref}
             onPlay={onPlay}
-            onScroll={onScroll}
             onCancel={onCancel}
             onClose={onClose}
           />
@@ -565,11 +559,6 @@ describe('ReactDOMEventListener', () => {
           bubbles: false,
         }),
       );
-      ref.current.dispatchEvent(
-        new Event('scroll', {
-          bubbles: false,
-        }),
-      );
       ref.current.dispatchEvent(
         new Event('cancel', {
           bubbles: false,
@@ -583,7 +572,6 @@ describe('ReactDOMEventListener', () => {
       // Regression test: ensure we still emulate bubbling with non-bubbling
       // media
       expect(onPlay).toHaveBeenCalledTimes(2);
-      expect(onScroll).toHaveBeenCalledTimes(2);
       expect(onCancel).toHaveBeenCalledTimes(2);
       expect(onClose).toHaveBeenCalledTimes(2);
     } finally {
@@ -634,4 +622,100 @@ describe('ReactDOMEventListener', () => {
       document.body.removeChild(container);
     }
   });
+
+  // We're moving towards aligning more closely with the browser.
+  // Currently we emulate bubbling for all non-bubbling events except scroll.
+  // We may expand this list in the future, removing emulated bubbling altogether.
+  it('should not emulate bubbling of scroll events', () => {
+    const container = document.createElement('div');
+    const ref = React.createRef();
+    const log = [];
+    const onScroll = jest.fn(e =>
+      log.push(['bubble', e.currentTarget.className]),
+    );
+    const onScrollCapture = jest.fn(e =>
+      log.push(['capture', e.currentTarget.className]),
+    );
+    document.body.appendChild(container);
+    try {
+      ReactDOM.render(
+        <div
+          className="grand"
+          onScroll={onScroll}
+          onScrollCapture={onScrollCapture}>
+          <div
+            className="parent"
+            onScroll={onScroll}
+            onScrollCapture={onScrollCapture}>
+            <div
+              className="child"
+              onScroll={onScroll}
+              onScrollCapture={onScrollCapture}
+              ref={ref}
+            />
+          </div>
+        </div>,
+        container,
+      );
+      ref.current.dispatchEvent(
+        new Event('scroll', {
+          bubbles: false,
+        }),
+      );
+      //
+      expect(log).toEqual([
+        ['capture', 'grand'],
+        ['capture', 'parent'],
+        ['capture', 'child'],
+        ['bubble', 'child'],
+      ]);
+    } finally {
+      document.body.removeChild(container);
+    }
+  });
+
+  // We're moving towards aligning more closely with the browser.
+  // Currently we emulate bubbling for all non-bubbling events except scroll.
+  // We may expand this list in the future, removing emulated bubbling altogether.
+  it('should not emulate bubbling of scroll events (no own handler)', () => {
+    const container = document.createElement('div');
+    const ref = React.createRef();
+    const log = [];
+    const onScroll = jest.fn(e =>
+      log.push(['bubble', e.currentTarget.className]),
+    );
+    const onScrollCapture = jest.fn(e =>
+      log.push(['capture', e.currentTarget.className]),
+    );
+    document.body.appendChild(container);
+    try {
+      ReactDOM.render(
+        <div
+          className="grand"
+          onScroll={onScroll}
+          onScrollCapture={onScrollCapture}>
+          <div
+            className="parent"
+            onScroll={onScroll}
+            onScrollCapture={onScrollCapture}>
+            {/* Intentionally no handler on the child: */}
+            <div className="child" ref={ref} />
+          </div>
+        </div>,
+        container,
+      );
+      ref.current.dispatchEvent(
+        new Event('scroll', {
+          bubbles: false,
+        }),
+      );
+      //
+      expect(log).toEqual([
+        ['capture', 'grand'],
+        ['capture', 'parent'],
+      ]);
+    } finally {
+      document.body.removeChild(container);
+    }
+  });
 });
diff --git a/packages/react-dom/src/events/plugins/SimpleEventPlugin.js b/packages/react-dom/src/events/plugins/SimpleEventPlugin.js
index 281065828322a..0bcb4dc2845c6 100644
--- a/packages/react-dom/src/events/plugins/SimpleEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/SimpleEventPlugin.js
@@ -165,13 +165,17 @@ function extractEvents(
       inCapturePhase,
     );
   } else {
-    // TODO: We may also want to re-use the accumulateTargetOnly flag to
-    // special case bubbling for onScroll/media events at a later point.
-    // In which case we will want to make this flag boolean and ensure
-    // we change the targetInst to be of the container instance. Like:
-    const accumulateTargetOnly = false;
+    // Some events don't bubble in the browser.
+    // In the past, React has always bubbled them, but this can be surprising.
+    // We're going to try aligning closer to the browser behavior by not bubbling
+    // them in React either. We'll start by not bubbling onScroll, and then expand.
+    const accumulateTargetOnly =
+      !inCapturePhase &&
+      // TODO: ideally, we'd eventually add all events from
+      // nonDelegatedEvents list in ReactDOMEventListener.
+      // Then we can remove this special list.
+      topLevelType === DOMTopLevelEventTypes.TOP_SCROLL;
 
-    // We traverse only capture or bubble phase listeners
     accumulateSinglePhaseListeners(
       targetInst,
       dispatchQueue,