feat(core): make Canvas focusable

The `Canvas` is now a focusable component, that is recognized
accordingly by the browser, with all benefits for UX and interaction.

Components that pull focus from the `Canvas` during modeling must
ensure to restore the focus (if intended), via `Canvas#restoreFocus`.

Related to #661

lib/core/Canvas.js

@@ -10,7 +10,8 @@ import {
 } from 'min-dash';
 
 import {
-  assignStyle
+  assignStyle,
+  attr as domAttr
 } from 'min-dom';
 
 import {
@@ -189,6 +190,11 @@ export default function Canvas(config, eventBus, graphicsFactory, elementRegistr
    */
   this._rootElement = null;
 
+  /**
+   * @type {boolean}
+   */
+  this._focused = false;
+
   this._init(config || {});
 }
 
@@ -215,14 +221,33 @@ Canvas.$inject = [
  * @param {CanvasConfig} config
  */
 Canvas.prototype._init = function(config) {
-
   const eventBus = this._eventBus;
 
   // html container
   const container = this._container = createContainer(config);
 
   const svg = this._svg = svgCreate('svg');
-  svgAttr(svg, { width: '100%', height: '100%' });
+
+  svgAttr(svg, {
+    width: '100%',
+    height: '100%'
+  });
+
+  domAttr(svg, 'tabindex', 0);
+
+  eventBus.on('element.hover', () => {
+    this.restoreFocus();
+  });
+
+  svg.addEventListener('focusin', () => {
+    this._focused = true;
+    eventBus.fire('canvas.focus.changed', { focused: true });
+  });
+
+  svg.addEventListener('focusout', () => {
+    this._focused = false;
+    eventBus.fire('canvas.focus.changed', { focused: false });
+  });
 
   svgAppend(container, svg);
 
@@ -314,6 +339,31 @@ Canvas.prototype._clear = function() {
   delete this._cachedViewbox;
 };
 
+/**
+* Sets focus on the canvas SVG element.
+*/
+Canvas.prototype.focus = function() {
+  this._svg.focus({ preventScroll: true });
+};
+
+/**
+* Sets focus on the canvas SVG element if `document.body` is currently focused.
+*/
+Canvas.prototype.restoreFocus = function() {
+  if (document.activeElement === document.body) {
+    this.focus();
+  }
+};
+
+/**
+* Returns true if the canvas is focused.
+*
+* @return {boolean}
+*/
+Canvas.prototype.isFocused = function() {
+  return this._focused;
+};
+
 /**
  * Returns the default layer on which
  * all elements are drawn.

lib/features/popup-menu/PopupMenu.js

@@ -262,6 +262,8 @@ PopupMenu.prototype.close = function() {
 
   this.reset();
 
+  this._canvas.restoreFocus();
+
   this._current = null;
 };
 

lib/features/search-pad/SearchPad.js

@@ -403,6 +403,8 @@ SearchPad.prototype.close = function(restoreCached = true) {
   this._searchInput.blur();
 
   this._eventBus.fire('searchPad.closed');
+
+  this._canvas.restoreFocus();
 };
 
 

test/spec/core/CanvasSpec.js

@@ -47,12 +47,18 @@ describe('Canvas', function() {
 
     beforeEach(createDiagram());
 
+
     it('should create <svg> element', inject(function() {
 
-      // then
+      // when
       var svg = container.querySelector('svg');
 
-      expect(svg).not.to.be.null;
+      // then
+      // svg is created
+      expect(svg).to.exist;
+
+      // and user selectable
+      expect(svgAttr(svg, 'tabindex')).to.equal('0');
     }));
 
 
@@ -152,6 +158,9 @@ describe('Canvas', function() {
 
         // then
         expect(document.activeElement).to.eql(inputEl);
+
+        // clean up
+        document.body.removeChild(inputEl);
       }));
 
     });

test/spec/features/popup-menu/PopupMenuSpec.js

@@ -706,6 +706,20 @@ describe('features/popup-menu', function() {
       }).not.to.throw();
     }));
 
+
+    it('should refocus canvas on close', inject(function(canvas, popupMenu) {
+
+      // given
+      sinon.spy(canvas, 'restoreFocus');
+
+      // when
+      popupMenu.close();
+
+      // then
+      expect(canvas.restoreFocus).to.have.been.calledOnce;
+
+    }));
+
   });
 
 

test/spec/features/search-pad/SearchPadSpec.js

@@ -277,6 +277,20 @@ describe('features/searchPad', function() {
       expect(capturedEvents).to.eql([ EVENTS.opened, EVENTS.closed ]);
     }));
 
+
+    it('should refocus canvas on close', inject(function(canvas, searchPad) {
+
+      // given
+      sinon.spy(canvas, 'restoreFocus');
+      searchPad.open();
+
+      // when
+      triggerMouseEvent(canvas.getContainer(), 'click', 0, 0);
+
+      // then
+      expect(canvas.restoreFocus).to.have.been.calledOnce;
+    }));
+
   });