(graphcache) - Fix offlineExchange timing after rehydration (#1019)

* Add experimental buffer fix for edge cases in Svelte/Preact

* Add changeset

.changeset/ninety-plants-peel.md

@@ -0,0 +1,5 @@
+---
+'@urql/exchange-graphcache': patch
+---
+
+Fix a case where the `offlineExchange` would not start processing operations after hydrating persisted data when no operations arrived in time by the time the persisted data was restored. This would be more evident in Preact and Svelte due to their internal short timings.

exchanges/graphcache/src/cacheExchange.ts

@@ -11,13 +11,14 @@ import {
 
 import {
   filter,
+  combine,
+  scan,
   map,
   merge,
   pipe,
   share,
   fromPromise,
   fromArray,
-  buffer,
   take,
   mergeMap,
   concat,
@@ -249,10 +250,15 @@ export const cacheExchange = (opts?: CacheExchangeOpts): Exchange => ({
     // If no hydration takes place we replace this stream with an empty one
     const bufferedOps$ = hydration
       ? pipe(
-          sharedOps$,
-          buffer(fromPromise(hydration)),
+          combine(
+            pipe(
+              sharedOps$,
+              scan((acc: Operation[], x) => (acc.push(x), acc), [])
+            ),
+            fromPromise(hydration)
+          ),
           take(1),
-          mergeMap(fromArray)
+          mergeMap(zip => fromArray(zip[0]))
         )
       : (empty as Source<Operation>);