fix(popper): fix popper gets covered by dialog/drawer (#861)

- Make popper's zIndex dynamic in order to allow popper related features in `Dialog/Drawer` to show

packages/popper/src/use-popper/index.ts

@@ -1,4 +1,4 @@
-import { computed, ref, reactive, watch } from 'vue'
+import { computed, ref, reactive, watch, CSSProperties } from 'vue'
 import { createPopper } from '@popperjs/core'
 
 import {
@@ -41,11 +41,7 @@ export default function(
 
   const isManualMode = () => props.manualMode || props.trigger === 'manual'
 
-  const popperStyle = computed(() => {
+  const popperStyle = ref<CSSProperties>({ zIndex: PopupManager.nextZIndex() })
-    return {
-      zIndex: String(PopupManager.nextZIndex()),
-    }
-  })
 
   const popperOptions = usePopperOptions(props, {
     arrow: arrowRef,
@@ -189,6 +185,7 @@ export default function(
 
   function onVisibilityChange(toState: boolean) {
     if (toState) {
+      popperStyle.value.zIndex = PopupManager.nextZIndex()
       initializePopper()
     }
   }