From c03c8478b0487b951a41d24dfbcb160cfaf3f8a7 Mon Sep 17 00:00:00 2001
From: IanM <ian@morland.me>
Date: Mon, 20 Apr 2026 22:53:13 +0100
Subject: [PATCH] fix: prevent DiscussionListState cache wipe on back-nav to
 tag pages
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DiscussionListState.requestParams returned `this.params.filter` by
reference. Extenders (tags, subscriptions) then mutated that object to
build the outgoing API filter, silently writing their additions back
into the stored state. On the next mount, the freshly-supplied filter
no longer equalled the mutated stored copy, so `paramsChanged()` tripped
and the paginated cache was wiped — the visible "back button resets to
page 1" symptom on tag pages (#4583).

Clone `filter` before handing it out so extender mutation lands on the
per-request object, not on `this.params`.
---
 .../src/forum/states/DiscussionListState.ts   |  5 +++-
 .../forum/states/DiscussionListState.test.ts  | 25 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 framework/core/js/tests/unit/forum/states/DiscussionListState.test.ts

diff --git a/framework/core/js/src/forum/states/DiscussionListState.ts b/framework/core/js/src/forum/states/DiscussionListState.ts
index f53f2b7514..aab272961b 100644
--- a/framework/core/js/src/forum/states/DiscussionListState.ts
+++ b/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(),
     };
 
diff --git a/framework/core/js/tests/unit/forum/states/DiscussionListState.test.ts b/framework/core/js/tests/unit/forum/states/DiscussionListState.test.ts
new file mode 100644
index 0000000000..f56dd0a4ba
--- /dev/null
+++ b/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();
+    });
+  });
+});