feat(history): preserve existing history.state

Closes #3006

Author: posva

src/util/push-state.js

@@ -3,6 +3,7 @@
 import { inBrowser } from './dom'
 import { saveScrollPosition } from './scroll'
 import { genStateKey, setStateKey, getStateKey } from './state-key'
+import { extend } from './misc'
 
 export const supportsPushState =
   inBrowser &&
@@ -28,7 +29,10 @@ export function pushState (url?: string, replace?: boolean) {
   const history = window.history
   try {
     if (replace) {
-      history.replaceState({ key: getStateKey() }, '', url)
+      // preserve existing history state as it could be overriden by the user
+      const stateCopy = extend({}, history.state)
+      stateCopy.key = getStateKey()
+      history.replaceState(stateCopy, '', url)
     } else {
       history.pushState({ key: setStateKey(genStateKey()) }, '', url)
     }