follow [tabindex]

the [tabindex] elements are used in gmail

src/content/components/common/follow.js

@@ -4,7 +4,7 @@ import * as dom from 'shared/utils/dom';
 
 const TARGET_SELECTOR = [
   'a', 'button', 'input', 'textarea',
-  '[contenteditable=true]', '[contenteditable=""]'
+  '[contenteditable=true]', '[contenteditable=""]', '[tabindex]'
 ].join(',');
 
 const inViewport = (win, element, viewSize, framePosition) => {
@@ -139,6 +139,8 @@ export default class Follow {
     default:
       if (dom.isContentEditable(element)) {
         return element.focus();
+      } else if (element.hasAttribute('tabindex')) {
+        return element.click();
       }
     }
   }