Fix hidden input when reselecting in single mode

What
---
- Removing hidden class from single inputs when the dropdown is hidden.
- Changing from `display:none` to zero width when hiding the input.

Why
---
Because the user will be typing for another element if reopening the
dropdown, which we need to allow.

src/components/Select.vue

@@ -215,7 +215,8 @@
     cursor: pointer;
   }
   .v-select input[type="search"].hidden {
-    display: none;
+    width: 0px;
+    padding: 0;
   }
   .v-select input[type="search"].shrunk {
     width: auto;
@@ -989,7 +990,7 @@
        */
       inputClasses() {
         return {
-          hidden: !this.multiple && !this.isValueEmpty,
+          hidden: !this.multiple && !this.isValueEmpty && !this.dropdownOpen,
           shrunk: this.multiple && !this.isValueEmpty,
           empty: this.isValueEmpty,
         }