[2.x] fix: prevent discussion list cache wipe on back-nav to tag pages

framework/core/js/src/forum/states/DiscussionListState.ts

@@ -25,9 +25,12 @@ export default class DiscussionListState<P extends DiscussionListParams = Discus
   }
 
   requestParams(): PaginatedListRequestParams {
+    // `filter` is cloned so extenders mutating it don't write back into
+    // `this.params.filter` — that leak trips `paramsChanged()` on the next
+    // mount and wipes the paginated cache (see #4583).
     const params = {
       include: ['user', 'lastPostedUser'],
-      filter: this.params.filter || {},
+      filter: { ...this.params.filter },
       sort: this.currentSort(),
     };
 

framework/core/js/tests/unit/forum/states/DiscussionListState.test.ts

@@ -0,0 +1,25 @@
+import bootstrapForum from '@flarum/jest-config/src/bootstrap/forum';
+import DiscussionListState from '../../../../src/forum/states/DiscussionListState';
+
+beforeAll(() => bootstrapForum());
+
+describe('DiscussionListState', () => {
+  describe('requestParams', () => {
+    // Regression test for #4583.
+    //
+    // Extenders (tags, subscriptions, …) mutate `params.filter` inside a
+    // `requestParams` extender callback. If `requestParams` returns
+    // `this.params.filter` by reference, those mutations leak back into the
+    // stored state — and on the next mount `paramsChanged()` falsely reports
+    // a change, wiping the paginated cache and resetting the list to page 1.
+    test('does not leak this.params.filter to callers', () => {
+      const state = new DiscussionListState({ filter: { tag: 'foo' } } as any);
+
+      const out = state.requestParams() as { filter: Record<string, string> };
+      out.filter.injectedByExtender = 'yes';
+
+      expect((state as any).params.filter).toEqual({ tag: 'foo' });
+      expect((state as any).params.filter.injectedByExtender).toBeUndefined();
+    });
+  });
+});