feat: use the new setup domain for lightning pages if available

Opening a `/lightning/` setup page on the instance domain caused
multiple server- and client-side redirects to the new setup domain.

This caused errors like:

> failed: Protocol error (DOM.describeNode): Cannot find context with specified id

Now all lightning pages should open using the new setup domain if available
and use the instance URL otherwise.

Example: https://[MyDomainName].my.salesforce-setup.com

src/browserforce.ts

@@ -13,6 +13,8 @@ export class Browserforce {
   public logger?: Ux;
   public browser: Browser;
   public page: Page;
+  public lightningSetupUrl: string;
+
   constructor(org: Org, logger?: Ux) {
     this.org = org;
     this.logger = logger;
@@ -62,7 +64,8 @@ export class Browserforce {
     const result = await pRetry(
       async () => {
         page = await this.getNewPage();
-        const url = `${this.getInstanceUrl()}/${urlPath}`;
+        const setupUrl = urlPath.startsWith("lightning") ? await this.getLightningSetupUrl() : this.getInstanceUrl();
+        const url = `${setupUrl}/${urlPath}`;
         const response = await page.goto(url, options);
         if (response) {
           if (!response.ok()) {
@@ -124,6 +127,22 @@ export class Browserforce {
     // sometimes the instanceUrl includes a trailing slash
     return this.org.getConnection().instanceUrl?.replace(/\/$/, '');
   }
+
+  /**
+   * @returns the setup url (e.g. https://[MyDomainName].my.salesforce-setup.com)
+   */
+  public async getLightningSetupUrl(): Promise<string> {
+    if (!this.lightningSetupUrl) {
+      const page = await this.getNewPage();
+      try {
+        const lightningResponse = await page.goto(`${this.getInstanceUrl()}/lightning/setup/SetupOneHome/home`, {waitUntil: ["load", "networkidle2"]});
+        this.lightningSetupUrl = new URL(lightningResponse.url()).origin;
+      } finally {
+        await page.close();
+      }
+    }
+    return this.lightningSetupUrl;
+  }
 }
 
 export async function throwPageErrors(page: Page): Promise<void> {

src/pages/login.ts

@@ -19,14 +19,14 @@ export class LoginPage {
       throw new Error('login failed');
     }
     const conn = org.getConnection();
-    await this.page.goto(
-      `${conn.instanceUrl}/${PATH}?sid=${conn.accessToken}&retURL=${encodeURIComponent(POST_LOGIN_PATH)}`,
+    const response = await this.page.goto(
+      `${conn.instanceUrl.replace(/\/$/, '')}/${PATH}?sid=${conn.accessToken}&retURL=${encodeURIComponent(POST_LOGIN_PATH)}`,
       {
         // should have waited at least 500ms for network connections, redirects should probably have happened already
         waitUntil: ['load', 'networkidle2']
       }
     );
-    const url = new URL(this.page.url());
+    const url = new URL(response.url());
     if (url.searchParams.has('startURL')) {
       // when query param startURL exists, the login failed
       // e.g. /?ec=302&startURL=https...

test/browserforce.e2e-spec.ts

@@ -33,9 +33,7 @@ describe('Browserforce', function () {
   });
   describe('waitForSelectorInFrameOrPage()', () => {
     it('should query a selector in LEX and Classic UI', async () => {
-      const page = await global.bf.openPage('lightning/setup/ExternalStrings/home', {
-        waitUntil: ['load', 'networkidle2']
-      });
+      const page = await global.bf.openPage('lightning/setup/ExternalStrings/home');
       const frame = await global.bf.waitForSelectorInFrameOrPage(page, 'input[name="edit"]');
       const button = await frame.$('input[name="edit"]');
       assert.ok(!page.url().includes('/page'));