fix: take margin into account (#291)

examples/switch.tsx

@@ -16,6 +16,7 @@ const MyItem: React.FC<Item> = ({ id }, ref) => (
       lineHeight: '30px',
       boxSizing: 'border-box',
       display: 'inline-block',
+      // marginBottom: 8,
     }}
   >
     {id}

src/hooks/useHeights.tsx

@@ -1,10 +1,15 @@
-import * as React from 'react';
-import { useRef, useEffect } from 'react';
 import findDOMNode from 'rc-util/lib/Dom/findDOMNode';
 import raf from 'rc-util/lib/raf';
+import * as React from 'react';
+import { useEffect, useRef } from 'react';
 import type { GetKey } from '../interface';
 import CacheMap from '../utils/CacheMap';
 
+function parseNumber(value: string) {
+  const num = parseFloat(value);
+  return isNaN(num) ? 0 : num;
+}
+
 export default function useHeights<T>(
   getKey: GetKey<T>,
   onItemAdd?: (item: T) => void,
@@ -32,8 +37,14 @@ export default function useHeights<T>(
         if (element && element.offsetParent) {
           const htmlElement = findDOMNode<HTMLElement>(element);
           const { offsetHeight } = htmlElement;
-          if (heightsRef.current.get(key) !== offsetHeight) {
-            heightsRef.current.set(key, htmlElement.offsetHeight);
+          const { marginTop, marginBottom } = getComputedStyle(htmlElement);
+
+          const marginTopNum = parseNumber(marginTop);
+          const marginBottomNum = parseNumber(marginBottom);
+          const totalHeight = offsetHeight + marginTopNum + marginBottomNum;
+
+          if (heightsRef.current.get(key) !== totalHeight) {
+            heightsRef.current.set(key, totalHeight);
           }
         }
       });