fix(components): [backtop] change default visible (#13069)

* fix(components): [backtop] change default visible

* fix(components): [backtop] handleScroll onMounted & add unit test

packages/components/backtop/__tests__/backtop.test.tsx

@@ -37,4 +37,12 @@ describe('Backtop.vue', () => {
     await wrapper.trigger('click')
     expect(wrapper.emitted()).toBeDefined()
   })
+  test('render when visibilityHeight is zero', async () => {
+    const wrapper = _mount(() => (
+      <Backtop visibilityHeight={0} right={100} bottom={200} />
+    ))
+    await nextTick()
+
+    expect(wrapper.find('.el-backtop').exists()).toBe(true)
+  })
 })

packages/components/backtop/src/use-backtop.ts

@@ -36,6 +36,8 @@ export const useBackTop = (
       }
       container.value = el.value
     }
+    // Give visible an initial value, fix #13066
+    handleScroll()
   })
 
   return {