From 0991207802ff0a87f1d5cd47ba4e86be5607c43a Mon Sep 17 00:00:00 2001
From: Astra <astra@zio.sh>
Date: Wed, 9 Sep 2026 13:53:20 +0100
Subject: [PATCH] forum: project the forum's own channel with member state in
 getForumTopics

messages.getForumTopics returned every chat via tgChannels -> tgChannelChatMin,
so the forum's own channel came back as a min object with left unset. A client
with no other object for that peer (a fresh account browsing a public forum by
username) then rendered the forum as already joined: topic list visible, no
Join button, but no messages.

Render the primary channel with tgChannelChatForView so a non-member preview
carries left=true; keep the other referenced channels as min.
---
 internal/rpc/forum_topics_preview_rpc_test.go | 15 +++++++++++
 internal/rpc/messages_forum.go                | 27 ++++++++++++++++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/internal/rpc/forum_topics_preview_rpc_test.go b/internal/rpc/forum_topics_preview_rpc_test.go
index d40f3d47..8bceefba 100644
--- a/internal/rpc/forum_topics_preview_rpc_test.go
+++ b/internal/rpc/forum_topics_preview_rpc_test.go
@@ -81,6 +81,21 @@ func TestGetForumTopicsVisibleToPublicNonMember(t *testing.T) {
 		t.Fatalf("non-member did not see the Test topic: %+v", res.Topics)
 	}
 
+	// The forum's own channel must come back with left=true so the client still
+	// offers a Join button instead of treating the forum as already joined.
+	var forumChat *tg.Channel
+	for _, c := range res.Chats {
+		if ch, ok := c.(*tg.Channel); ok && ch.ID == channel.ID {
+			forumChat = ch
+		}
+	}
+	if forumChat == nil {
+		t.Fatalf("forum channel missing from getForumTopics chats: %+v", res.Chats)
+	}
+	if !forumChat.Left {
+		t.Fatalf("non-member forum chat = %#v, want left=true", forumChat)
+	}
+
 	// A private forum still refuses a non-member.
 	priv, err := r.onChannelsCreateChannel(ownerCtx, &tg.ChannelsCreateChannelRequest{Title: "Private Forum", Megagroup: true})
 	if err != nil {
diff --git a/internal/rpc/messages_forum.go b/internal/rpc/messages_forum.go
index 4a0e939a..03702495 100644
--- a/internal/rpc/messages_forum.go
+++ b/internal/rpc/messages_forum.go
@@ -474,12 +474,37 @@ func (r *Router) forumTopicsResponse(ctx context.Context, userID int64, view dom
 		Count:    count,
 		Topics:   topics,
 		Messages: messages,
-		Chats:    tgChannels(userID, channels),
+		Chats:    r.forumTopicsChats(userID, view, channels),
 		Users:    r.tgUsersForIDs(ctx, userID, userIDs),
 		Pts:      view.Channel.Pts,
 	})
 }
 
+// forumTopicsChats projects the forum's own channel with the viewer's member
+// state (so a non-member preview carries left=true and the client still shows a
+// Join button) and every other referenced channel as a min chat. Rendering the
+// primary as a bare min chat lets a client that has no other object for the
+// channel treat the forum as already joined.
+func (r *Router) forumTopicsChats(userID int64, view domain.ChannelView, channels []domain.Channel) []tg.ChatClass {
+	if view.Channel.ID == 0 {
+		return tgChannels(userID, channels)
+	}
+	chats := make([]tg.ChatClass, 0, len(channels))
+	chats = append(chats, tgChannelChatForView(userID, view))
+	seen := map[int64]struct{}{view.Channel.ID: {}}
+	for _, extra := range channels {
+		if extra.ID == 0 {
+			continue
+		}
+		if _, dup := seen[extra.ID]; dup {
+			continue
+		}
+		seen[extra.ID] = struct{}{}
+		chats = append(chats, tgChannelChatMin(userID, extra))
+	}
+	return chats
+}
+
 func tgForumGeneralTopic(viewerUserID int64, view domain.ChannelView, topic domain.ChannelForumTopic) *tg.ForumTopic {
 	return &tg.ForumTopic{
 		My:                   view.Channel.CreatorUserID == viewerUserID && viewerUserID != 0,