router restore should take priority over pending actions (#64449)

Since the router processes events sequentially, we special case
`ACTION_NAVIGATE` events to "discard" a pending server action so that it
can process the navigation with higher priority. We should apply this
same logic to `ACTION_RESTORE` events (e.g. `router.back()`) for the
same reason.

Fixes #64432


Closes NEXT-3098

Author: ztanner

packages/next/src/shared/lib/router/action-queue.ts

@@ -155,8 +155,11 @@ function dispatchAction(
       action: newAction,
       setState,
     })
-  } else if (payload.type === ACTION_NAVIGATE) {
-    // Navigations take priority over any pending actions.
+  } else if (
+    payload.type === ACTION_NAVIGATE ||
+    payload.type === ACTION_RESTORE
+  ) {
+    // Navigations (including back/forward) take priority over any pending actions.
     // Mark the pending action as discarded (so the state is never applied) and start the navigation action immediately.
     actionQueue.pending.discarded = true
 

test/e2e/app-dir/actions/app-action.test.ts

@@ -337,6 +337,19 @@ createNextDescribe(
       await check(() => browser.url(), `${next.url}/client`, true, 2)
     })
 
+    it('should not block router.back() while a server action is in flight', async () => {
+      let browser = await next.browser('/')
+
+      // click /client link to add a history entry
+      await browser.elementByCss("[href='/client']").click()
+      await browser.elementByCss('#slow-inc').click()
+
+      await browser.back()
+
+      // intentionally bailing after 2 retries so we don't retry to the point where the async function resolves
+      await check(() => browser.url(), `${next.url}/`, true, 2)
+    })
+
     it('should trigger a refresh for a server action that gets discarded due to a navigation', async () => {
       let browser = await next.browser('/client')
       const initialRandomNumber = await browser