Fall back to the first sign-in tab if the local storage value is bad

If `current_signin_tab` is `#crowd`, then no tab will be active. Fix
this by using the first available tab.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20049#note_82996324

app/assets/javascripts/pages/sessions/new/signin_tabs_memoizer.js

@@ -37,6 +37,11 @@ export default class SigninTabsMemoizer {
       const tab = document.querySelector(`${this.tabSelector} a[href="${anchorName}"]`);
       if (tab) {
         tab.click();
+      } else {
+        const firstTab = document.querySelector(`${this.tabSelector} a`);
+        if (firstTab) {
+          firstTab.click();
+        }
       }
     }
   }

spec/javascripts/signin_tabs_memoizer_spec.js

@@ -45,6 +45,21 @@ import SigninTabsMemoizer from '~/pages/sessions/new/signin_tabs_memoizer';
       expect(fakeTab.click).toHaveBeenCalled();
     });
 
+    it('clicks the first tab if value in local storage is bad', () => {
+      createMemoizer().saveData('#bogus');
+      const fakeTab = {
+        click: () => {},
+      };
+      spyOn(document, 'querySelector').and.callFake(selector => (selector === `${tabSelector} a[href="#bogus"]` ? null : fakeTab));
+      spyOn(fakeTab, 'click');
+
+      memo.bootstrap();
+
+      // verify that triggers click on stored selector and fallback
+      expect(document.querySelector.calls.allArgs()).toEqual([['ul.new-session-tabs a[href="#bogus"]'], ['ul.new-session-tabs a']]);
+      expect(fakeTab.click).toHaveBeenCalled();
+    });
+
     it('saves last selected tab on change', () => {
       createMemoizer();