[Scrollable] fix: scrollable calls onScrolledToBottom only when container is overflowing (#7635)

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### WHY are these changes introduced?

Fixes #0000 <!-- link to issue if one exists -->

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### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#local-development)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

In storybook, create a scrollable component with a small amount of text
so that it doesn't scroll. Make sure the `onScrolledToBottom` callback
is **not** fired when the scrollable component renders.

On `main` the opposite should be true.

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: iAmNathanJ

.changeset/thick-clocks-applaud.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Fixed Scrollable component to match existing onScrolledToBottom logic

polaris-react/src/components/Scrollable/Scrollable.tsx

@@ -67,6 +67,7 @@ export function Scrollable({
 
     requestAnimationFrame(() => {
       const {scrollTop, clientHeight, scrollHeight} = currentScrollArea;
+      const canScroll = Boolean(scrollHeight > clientHeight);
       const isBelowTopOfScroll = Boolean(scrollTop > 0);
       const isAtBottomOfScroll = Boolean(
         scrollTop + clientHeight >= scrollHeight - LOW_RES_BUFFER,
@@ -75,7 +76,7 @@ export function Scrollable({
       setTopShadow(isBelowTopOfScroll);
       setBottomShadow(!isAtBottomOfScroll);
 
-      if (isAtBottomOfScroll && onScrolledToBottom) {
+      if (canScroll && isAtBottomOfScroll && onScrolledToBottom) {
         onScrolledToBottom();
       }
     });

polaris-react/src/components/Scrollable/tests/Scrollable.test.tsx

@@ -3,8 +3,24 @@ import {mountWithApp} from 'tests/utilities';
 
 import {Scrollable} from '../Scrollable';
 import {ScrollableContext} from '../context';
+import type {ScrollableProps} from '../Scrollable';
 
 describe('<Scrollable />', () => {
+  let rafSpy: jest.SpyInstance;
+
+  beforeAll(() => {
+    rafSpy = jest
+      .spyOn(window, 'requestAnimationFrame')
+      .mockImplementation((cb) => {
+        cb(Date.now());
+        return Math.random();
+      });
+  });
+
+  afterAll(() => {
+    rafSpy.mockRestore();
+  });
+
   it('mounts', () => {
     const scrollable = mountWithApp(<Scrollable />);
     expect(scrollable).toBeDefined();
@@ -71,4 +87,60 @@ describe('<Scrollable />', () => {
       tabIndex: 0,
     });
   });
+
+  it('calls onScrolledToBottom when scrolled to bottom', () => {
+    const onScrolledToBottom = jest.fn();
+
+    const scrollArea = mountWithApp(
+      <Scrollable
+        data-test-id="scroll-element"
+        onScrolledToBottom={onScrolledToBottom}
+      >
+        <p>Hello</p>
+      </Scrollable>,
+    );
+
+    const scrollNode = scrollArea.find('div', {
+      'data-test-id': 'scroll-element',
+    } as ScrollableProps)?.domNode!;
+
+    // defineProperty needed to assign values to readonly node properties
+    Object.defineProperty(scrollNode, 'clientHeight', {get: () => 0});
+    Object.defineProperty(scrollNode, 'scrollHeight', {get: () => 10});
+    Object.defineProperty(scrollNode, 'scrollTop', {get: () => 10});
+
+    scrollArea.act(() => {
+      scrollNode.dispatchEvent(new Event('scroll'));
+    });
+
+    expect(onScrolledToBottom).toHaveBeenCalledTimes(1);
+  });
+
+  it(`doesn't call onScrolledToBottom when the scroll area is not overflowing`, () => {
+    const onScrolledToBottom = jest.fn();
+
+    const scrollArea = mountWithApp(
+      <Scrollable
+        data-test-id="scroll-element"
+        onScrolledToBottom={onScrolledToBottom}
+      >
+        <p>Hello</p>
+      </Scrollable>,
+    );
+
+    const scrollNode = scrollArea.find('div', {
+      'data-test-id': 'scroll-element',
+    } as ScrollableProps)?.domNode!;
+
+    // defineProperty needed to assign values to readonly node properties
+    Object.defineProperty(scrollNode, 'clientHeight', {get: () => 10});
+    Object.defineProperty(scrollNode, 'scrollHeight', {get: () => 10});
+    Object.defineProperty(scrollNode, 'scrollTop', {get: () => 10});
+
+    scrollArea.act(() => {
+      scrollNode.dispatchEvent(new Event('scroll'));
+    });
+
+    expect(onScrolledToBottom).not.toHaveBeenCalled();
+  });
 });