fix: Motion should provider a basic hidden className

README.md

@@ -180,7 +180,7 @@ If `accordion` is true, only one panel can be open.  Opening another panel will
         <td>openMotion</td>
         <td>object</td>
         <th></th>
-        <td>set the animation of open behavior, [more](https://github.com/react-component/motion)</td>
+        <td>set the animation of open behavior, [more](https://github.com/react-component/motion). Different with v2, closed pane use a `rc-collapse-content-hidden` class to set `display: none` for hidden.</td>
       </tr>
       <tr>
         <td>disabled</td>

src/Panel.tsx

@@ -95,6 +95,7 @@ class CollapsePanel extends React.Component<CollapsePanelProps, any> {
         </div>
         <CSSMotion
           visible={isActive}
+          leavedClassName={`${prefixCls}-content-hidden`}
           {...openMotion}
           forceRender={forceRender}
           removeOnLeave={destroyInactivePanel}

tests/index.spec.tsx

@@ -358,6 +358,7 @@ describe('collapse', () => {
       );
       expect(collapse.find('.rc-collapse-content').length).toBe(1);
       expect(collapse.find('.rc-collapse-content-active').length).toBe(0);
+      expect(collapse.find('div.rc-collapse-content-hidden').length).toBe(1);
     });
   });