Update dropdown.vue

It is recommended to turn off scroll event monitoring for popper plug-in configuration, otherwise, if there are too many such components in a single page, the scroll bar of the page will be severely stuck; in fact, it only needs to be updated when the pop-up layer is displayed, and it does not need to be invisible or to update the position.

src/components/select/dropdown.vue

@@ -52,6 +52,7 @@
                 } else {
                     this.$nextTick(() => {
                         this.popper = new Popper(this.$parent.$refs.reference, this.$el, {
+                            eventsEnabled: false,
                             placement: this.placement,
                             modifiers: {
                                 computeStyle:{