Added "tabindex=0" for sidebar accessibility (#583)

* Added tabindex=0 for screen reader accessibility

* modified createTabARIA to include BaseAttrs; removed duplicate of tabindex in virtualdom

* Update API summary

---------

Co-authored-by: krassowski <5832902+krassowski@users.noreply.github.com>

packages/widgets/src/tabbar.ts

@@ -21,6 +21,7 @@ import { ISignal, Signal } from '@lumino/signaling';
 
 import {
   ElementARIAAttrs,
+  ElementBaseAttrs,
   ElementDataset,
   ElementInlineStyle,
   h,
@@ -1666,8 +1667,12 @@ export namespace TabBar {
      *
      * @returns The ARIA attributes for the tab.
      */
-    createTabARIA(data: IRenderData<any>): ElementARIAAttrs {
-      return { role: 'tab', 'aria-selected': data.current.toString() };
+    createTabARIA(data: IRenderData<any>): ElementARIAAttrs | ElementBaseAttrs {
+      return {
+        role: 'tab',
+        'aria-selected': data.current.toString(),
+        tabindex: '0'
+      };
     }
 
     /**

review/api/widgets.api.md

@@ -7,6 +7,7 @@
 import { CommandRegistry } from '@lumino/commands';
 import { ConflatableMessage } from '@lumino/messaging';
 import { ElementARIAAttrs } from '@lumino/virtualdom';
+import { ElementBaseAttrs } from '@lumino/virtualdom';
 import { ElementDataset } from '@lumino/virtualdom';
 import { ElementInlineStyle } from '@lumino/virtualdom';
 import { h } from '@lumino/virtualdom';
@@ -1151,7 +1152,7 @@ export namespace TabBar {
         constructor();
         readonly closeIconSelector = ".lm-TabBar-tabCloseIcon";
         createIconClass(data: IRenderData<any>): string;
-        createTabARIA(data: IRenderData<any>): ElementARIAAttrs;
+        createTabARIA(data: IRenderData<any>): ElementARIAAttrs | ElementBaseAttrs;
         createTabClass(data: IRenderData<any>): string;
         createTabDataset(data: IRenderData<any>): ElementDataset;
         createTabKey(data: IRenderData<any>): string;