Reject mismatched reply route roots

Keep malformed reply intent non-actionable even when the selected reply and its actual thread root are already loaded.

Co-authored-by: Rivet <a08d9a8418c7ff03afe19964724c8fd87bf1776ab9e9b9cafb8cc920edd02a6e@buzz.block.builderlab.xyz>
Signed-off-by: Rivet <a08d9a8418c7ff03afe19964724c8fd87bf1776ab9e9b9cafb8cc920edd02a6e@buzz.block.builderlab.xyz>

desktop/src/features/channels/ui/useChannelRouteTarget.test.mjs

@@ -48,7 +48,7 @@ test("top-level target with an explicit threadRootId opens its thread panel", ()
   });
 });
 
-test("reply target opens the thread scrolled to the reply", () => {
+test("reply target without threadRootId opens the derived thread", () => {
   const root = makeMessage({ id: "root" });
   const reply = makeMessage({
     id: "reply",
@@ -65,6 +65,40 @@ test("reply target opens the thread scrolled to the reply", () => {
   });
 });
 
+test("reply target with its derived threadRootId opens the thread", () => {
+  const root = makeMessage({ id: "root" });
+  const reply = makeMessage({
+    id: "reply",
+    parentId: "root",
+    rootId: "root",
+    depth: 1,
+  });
+  assert.deepEqual(
+    getRouteTargetPanelAction(reply, root.id, byId(root, reply)),
+    {
+      kind: "open-thread",
+      expandedReplyIds: new Set(),
+      replyTargetId: "root",
+      scrollTargetId: "reply",
+      threadHeadId: "root",
+    },
+  );
+});
+
+test("reply target with a mismatched threadRootId does not open the loaded derived thread", () => {
+  const root = makeMessage({ id: "root" });
+  const reply = makeMessage({
+    id: "reply",
+    parentId: "root",
+    rootId: "root",
+    depth: 1,
+  });
+  assert.deepEqual(
+    getRouteTargetPanelAction(reply, "unrelated-root", byId(root, reply)),
+    { kind: "none" },
+  );
+});
+
 test("nested reply target expands its intermediate ancestors", () => {
   const root = makeMessage({ id: "root" });
   const mid = makeMessage({

desktop/src/features/channels/ui/useChannelRouteTarget.ts

@@ -84,6 +84,11 @@ export function getRouteTargetPanelAction(
     };
   }
 
+  const derivedRootId = targetMessage.rootId ?? targetMessage.parentId;
+  if (targetThreadRootId !== null && targetThreadRootId !== derivedRootId) {
+    return { kind: "none" };
+  }
+
   if (isBroadcastReply(targetMessage.tags ?? [])) {
     return { kind: "none" };
   }