feat(transition): handle transition classes when patching classes

packages/runtime-dom/src/modules/class.ts

@@ -1,7 +1,17 @@
+import { ElementWithTransition } from '../components/CSSTransition'
+
 // compiler should normalize class + :class bindings on the same element
 // into a single binding ['staticClass', dynamic]
-
-export function patchClass(el: Element, value: string, isSVG: boolean) {
+export function patchClass(
+  el: ElementWithTransition,
+  value: string,
+  isSVG: boolean
+) {
+  // if this is an element during a transition, take the temporary transition
+  // classes into account.
+  if (el._vtc) {
+    value = [value, ...el._vtc].join(' ')
+  }
   // directly setting className should be faster than setAttribute in theory
   if (isSVG) {
     el.setAttribute('class', value)