Add Skip Link to Notebook (#6844)

* Update base.css

* Update shell.ts

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* prettier lint test fixes

* Fix ESLint error

* Update tests

* Update more tests

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jeremy Tuloup <jeremy.tuloup@gmail.com>

packages/application/src/shell.ts

@@ -116,6 +116,13 @@ export class NotebookShell extends Widget implements JupyterFrontEnd.IShell {
     rootLayout.addWidget(hsplitPanel);
 
     this.layout = rootLayout;
+
+    // Added Skip to Main Link
+    const skipLinkWidgetHandler = (this._skipLinkWidgetHandler =
+      new Private.SkipLinkWidgetHandler(this));
+
+    this.add(skipLinkWidgetHandler.skipLinkWidget, 'top', { rank: 0 });
+    this._skipLinkWidgetHandler.show();
   }
 
   /**
@@ -349,6 +356,7 @@ export class NotebookShell extends Widget implements JupyterFrontEnd.IShell {
   private _rightHandler: SidePanelHandler;
   private _spacer_top: Widget;
   private _spacer_bottom: Widget;
+  private _skipLinkWidgetHandler: Private.SkipLinkWidgetHandler;
   private _main: Panel;
   private _translator: ITranslator = nullTranslator;
   private _currentChanged = new Signal<this, void>(this);
@@ -364,3 +372,82 @@ export namespace Shell {
    */
   export type Area = 'main' | 'top' | 'left' | 'right' | 'menu';
 }
+
+export namespace Private {
+  export class SkipLinkWidgetHandler {
+    /**
+     * Construct a new skipLink widget handler.
+     */
+    constructor(shell: INotebookShell) {
+      const skipLinkWidget = (this._skipLinkWidget = new Widget());
+      const skipToMain = document.createElement('a');
+      skipToMain.href = '#first-cell';
+      skipToMain.tabIndex = 1;
+      skipToMain.text = 'Skip to Main';
+      skipToMain.className = 'skip-link';
+      skipToMain.addEventListener('click', this);
+      skipLinkWidget.addClass('jp-skiplink');
+      skipLinkWidget.id = 'jp-skiplink';
+      skipLinkWidget.node.appendChild(skipToMain);
+    }
+
+    handleEvent(event: Event): void {
+      switch (event.type) {
+        case 'click':
+          this._focusMain();
+          break;
+      }
+    }
+
+    private _focusMain() {
+      const input = document.querySelector(
+        '#main-panel .jp-InputArea-editor'
+      ) as HTMLInputElement;
+      input.tabIndex = 1;
+      input.focus();
+    }
+
+    /**
+     * Get the input element managed by the handler.
+     */
+    get skipLinkWidget(): Widget {
+      return this._skipLinkWidget;
+    }
+
+    /**
+     * Dispose of the handler and the resources it holds.
+     */
+    dispose(): void {
+      if (this.isDisposed) {
+        return;
+      }
+      this._isDisposed = true;
+      this._skipLinkWidget.node.removeEventListener('click', this);
+      this._skipLinkWidget.dispose();
+    }
+
+    /**
+     * Hide the skipLink widget.
+     */
+    hide(): void {
+      this._skipLinkWidget.hide();
+    }
+
+    /**
+     * Show the skipLink widget.
+     */
+    show(): void {
+      this._skipLinkWidget.show();
+    }
+
+    /**
+     * Test whether the handler has been disposed.
+     */
+    get isDisposed(): boolean {
+      return this._isDisposed;
+    }
+
+    private _skipLinkWidget: Widget;
+    private _isDisposed = false;
+  }
+}

packages/application/test/shell.spec.ts

@@ -28,12 +28,17 @@ describe('Shell for notebooks', () => {
       expect(shell).toBeInstanceOf(NotebookShell);
     });
 
-    it('should make all areas empty initially', () => {
-      ['main', 'top', 'left', 'right', 'menu'].forEach((area) => {
+    it('should make some areas empty initially', () => {
+      ['main', 'left', 'right', 'menu'].forEach((area) => {
         const widgets = Array.from(shell.widgets(area as Shell.Area));
         expect(widgets.length).toEqual(0);
       });
     });
+
+    it('should have the skip link widget in the top area initially', () => {
+      const widgets = Array.from(shell.widgets('top'));
+      expect(widgets.length).toEqual(1);
+    });
   });
 
   describe('#widgets()', () => {
@@ -132,12 +137,17 @@ describe('Shell for tree view', () => {
       expect(shell).toBeInstanceOf(NotebookShell);
     });
 
-    it('should make all areas empty initially', () => {
-      ['main', 'top', 'left', 'right', 'menu'].forEach((area) => {
+    it('should make some areas empty initially', () => {
+      ['main', 'left', 'right', 'menu'].forEach((area) => {
         const widgets = Array.from(shell.widgets(area as Shell.Area));
         expect(widgets.length).toEqual(0);
       });
     });
+
+    it('should have the skip link widget in the top area initially', () => {
+      const widgets = Array.from(shell.widgets('top'));
+      expect(widgets.length).toEqual(1);
+    });
   });
 
   describe('#widgets()', () => {

packages/notebook-extension/style/base.css

@@ -280,3 +280,27 @@ body[data-notebook='notebooks']
   overflow: hidden;
   white-space: nowrap;
 }
+
+.jp-skiplink {
+  position: absolute;
+  top: -100em;
+}
+
+.jp-skiplink:focus-within {
+  position: absolute;
+  z-index: 10000;
+  top: 0;
+  left: 46%;
+  margin: 0 auto;
+  padding: 1em;
+  width: 15%;
+  box-shadow: var(--jp-elevation-z4);
+  border-radius: 4px;
+  background: var(--jp-layout-color0);
+  text-align: center;
+}
+
+.jp-skiplink:focus-within a {
+  text-decoration: underline;
+  color: var(--jp-content-link-color);
+}