feat(Sticky): offset-top support rem unit (#6556)

src/sticky/test/__snapshots__/index.spec.js.snap

@@ -28,6 +28,14 @@ exports[`offset-top prop 1`] = `
 </div>
 `;
 
+exports[`offset-top with rem unit 1`] = `
+<div style="height: 10px;">
+  <div class="van-sticky van-sticky--fixed" style="top: 16px;">
+    Content
+  </div>
+</div>
+`;
+
 exports[`sticky to top 1`] = `
 <div style="height: 10px;">
   <div class="van-sticky">

src/sticky/test/index.spec.js

@@ -43,6 +43,26 @@ test('offset-top prop', () => {
   mockScrollTop(0);
 });
 
+test('offset-top with rem unit', () => {
+  const originGetComputedStyle = window.getComputedStyle;
+
+  window.getComputedStyle = () => ({ fontSize: '16px' });
+
+  const wrapper = mount({
+    template: `
+      <van-sticky style="height: 10px;" offset-top="1rem">
+        Content
+      </van-sticky>
+    `,
+  });
+
+  mockScrollTop(100);
+  expect(wrapper).toMatchSnapshot();
+  mockScrollTop(0);
+
+  window.getComputedStyle = originGetComputedStyle;
+});
+
 test('should not trigger scroll event when hidden', () => {
   const scroll = jest.fn();
   mount({