fix(backend-registry): preserve URL fragments in withBackendSelectionParams (#16619)

Co-authored-by: VascoSch92 <vasco.schiavo@protonmail.com>

__tests__/api/backend-registry/url-selection.test.ts

@@ -65,6 +65,73 @@ describe("withBackendSelectionParams", () => {
       `/conversations/abc?tab=files&${BACKEND_QUERY_PARAM}=local-1`,
     );
   });
+
+  it("keeps a fragment after existing query parameters intact and after the query", () => {
+    const path = withBackendSelectionParams(
+      "/conversations/abc?tab=files#detail",
+      {
+        backend: localBackend,
+        orgId: null,
+      },
+    );
+
+    expect(path).toBe(
+      `/conversations/abc?tab=files&${BACKEND_QUERY_PARAM}=local-1#detail`,
+    );
+  });
+
+  it("keeps a fragment on a path without query parameters after the query", () => {
+    const path = withBackendSelectionParams("/conversations/abc#detail", {
+      backend: localBackend,
+      orgId: null,
+    });
+
+    expect(path).toBe(
+      `/conversations/abc?${BACKEND_QUERY_PARAM}=local-1#detail`,
+    );
+  });
+
+  it("does not treat a ? inside the fragment as a query separator", () => {
+    const path = withBackendSelectionParams("/conversations/abc#detail?x=1", {
+      backend: localBackend,
+      orgId: null,
+    });
+
+    expect(path).toBe(
+      `/conversations/abc?${BACKEND_QUERY_PARAM}=local-1#detail?x=1`,
+    );
+  });
+
+  it("round-trips an empty fragment verbatim", () => {
+    const path = withBackendSelectionParams("/conversations/abc#", {
+      backend: localBackend,
+      orgId: null,
+    });
+
+    expect(path).toBe(`/conversations/abc?${BACKEND_QUERY_PARAM}=local-1#`);
+  });
+
+  it("keeps the org id and the fragment together", () => {
+    const path = withBackendSelectionParams("/conversations/abc#detail", {
+      backend: cloudBackend,
+      orgId: "org-7",
+    });
+
+    expect(path).toBe(
+      `/conversations/abc?${BACKEND_QUERY_PARAM}=prod&${ORG_QUERY_PARAM}=org-7#detail`,
+    );
+  });
+
+  it("keeps query data that itself contains a ?", () => {
+    const path = withBackendSelectionParams("/conversations/abc?next=/a?b=1", {
+      backend: localBackend,
+      orgId: null,
+    });
+
+    expect(path).toBe(
+      `/conversations/abc?next=%2Fa%3Fb%3D1&${BACKEND_QUERY_PARAM}=local-1`,
+    );
+  });
 });
 
 describe("readBackendSelectionFromUrl", () => {

src/api/backend-registry/url-selection.ts

@@ -19,7 +19,8 @@ export const ORG_QUERY_PARAM = "org";
 
 /**
  * Append the active backend identity to an in-app path so opening it in a new
- * browsing context resolves against the same backend.
+ * browsing context resolves against the same backend. Any fragment on the
+ * path is preserved verbatim and kept after the query string.
  */
 export function withBackendSelectionParams(
   path: string,
@@ -28,12 +29,24 @@ export function withBackendSelectionParams(
   const { backend, orgId } = active;
   if (!backend.id) return path;
 
-  const [pathname, existingSearch = ""] = path.split("?");
+  const hashIndex = path.indexOf("#");
+  const fragment = hashIndex === -1 ? "" : path.slice(hashIndex);
+  const withoutFragment = hashIndex === -1 ? path : path.slice(0, hashIndex);
+
+  // Split at the *first* `?` only. A later `?` is ordinary query data (a
+  // `next=` redirect carrying its own query, say), and `split("?")` would
+  // silently drop everything past it.
+  const queryIndex = withoutFragment.indexOf("?");
+  const pathname =
+    queryIndex === -1 ? withoutFragment : withoutFragment.slice(0, queryIndex);
+  const existingSearch =
+    queryIndex === -1 ? "" : withoutFragment.slice(queryIndex + 1);
+
   const params = new URLSearchParams(existingSearch);
   params.set(BACKEND_QUERY_PARAM, backend.id);
   if (orgId) params.set(ORG_QUERY_PARAM, orgId);
 
-  return `${pathname}?${params.toString()}`;
+  return `${pathname}?${params.toString()}${fragment}`;
 }
 
 /**