fix(frame-router): fix double-slash in host URL processing when path is /

The way we process host URLs before passing to the client triggers a corner case where the native
URL type silently ignores certain changes to the `pathname` field.  This results in a bug where a
path of `/` is represented to client apps as `//`.

COMUI-4108

Author: MattCheely

packages/iframe-coordinator/src/elements/frame-router.ts

@@ -272,7 +272,9 @@ export default class FrameRouterElement extends HTMLElement {
     if (!hostUrlObject.hash) {
       hostUrlObject.pathname = stripTrailingSlash(hostUrlObject.pathname);
       if (window.location.hash) {
-        hostUrlObject.pathname += "/";
+        if (hostUrlObject.pathname !== "/") {
+          hostUrlObject.pathname += "/";
+        }
         hostUrlObject.hash = "#";
       }
     }

packages/iframe-coordinator/src/specs/FrameRouter.spec.ts

@@ -5,6 +5,11 @@ const ENV_DATA = {
   hostRootUrl: "https://example.com/root/",
 };
 
+const ENV_DATA_SINGLE_SLASH = {
+  ...ENV_DATA,
+  hostRootUrl: "https://example.com/",
+};
+
 const ENV_DATA_WITH_HASH = {
   ...ENV_DATA,
   hostRootUrl: "https://example.com/root/#/",
@@ -62,6 +67,33 @@ describe("The frame router element", () => {
       });
     });
 
+    it("Correctly process a host URL using the root path with no hash", () => {
+      const router = new FrameRouterElement();
+      router.clientConfig = {
+        clients: {},
+        envData: ENV_DATA_SINGLE_SLASH,
+      };
+      //@ts-ignore
+      expect(router._envData).toEqual({
+        ...ENV_DATA,
+        hostRootUrl: "https://example.com/",
+      });
+    });
+
+    it("Correctly process a host URL using the root path with a hash", () => {
+      window.location.hash = "foo";
+      const router = new FrameRouterElement();
+      router.clientConfig = {
+        clients: {},
+        envData: ENV_DATA_SINGLE_SLASH,
+      };
+      //@ts-ignore
+      expect(router._envData).toEqual({
+        ...ENV_DATA,
+        hostRootUrl: "https://example.com/#",
+      });
+    });
+
     it("Does not modify the provided host URL if it and the current location have a fragment", () => {
       window.location.hash = "foo";
       const router = new FrameRouterElement();