Implement side-by-side mode for VitalSource EPUB books

In the VitalSource integration, enable the side-by-side mode in the
underlying HTML document integration.

src/annotator/integrations/test/vitalsource-test.js

@@ -67,7 +67,9 @@ describe('annotator/integrations/vitalsource', () => {
       contentContainer: sinon.stub(),
       describe: sinon.stub(),
       destroy: sinon.stub(),
+      fitSideBySide: sinon.stub().returns(false),
       scrollToAnchor: sinon.stub(),
+      sideBySideEnabled: false,
     };
 
     FakeHTMLIntegration = sinon.stub().returns(fakeHTMLIntegration);
@@ -189,9 +191,16 @@ describe('annotator/integrations/vitalsource', () => {
       assert.equal(integration.canAnnotate(), true);
     });
 
-    it('does not support side-by-side mode', () => {
+    it('delegates to HTMLIntegration for side-by-side mode', () => {
       const integration = createIntegration();
-      assert.equal(integration.fitSideBySide(), false);
+      fakeHTMLIntegration.fitSideBySide.returns(true);
+      assert.isTrue(fakeHTMLIntegration.sideBySideEnabled);
+
+      const layout = { expanded: true, width: 150 };
+      const isActive = integration.fitSideBySide(layout);
+
+      assert.isTrue(isActive);
+      assert.calledWith(fakeHTMLIntegration.fitSideBySide, layout);
     });
 
     it('stops mouse events from propagating to parent frame', () => {

src/annotator/integrations/vitalsource.js

@@ -8,6 +8,7 @@ import { injectClient } from '../hypothesis-injector';
  * @typedef {import('../../types/annotator').Annotator} Annotator
  * @typedef {import('../../types/annotator').Integration} Integration
  * @typedef {import('../../types/annotator').Selector} Selector
+ * @typedef {import('../../types/annotator').SidebarLayout} SidebarLayout
  */
 
 /**
@@ -146,6 +147,7 @@ export class VitalSourceContentIntegration {
    */
   constructor(container = document.body) {
     this._htmlIntegration = new HTMLIntegration(container);
+    this._htmlIntegration.sideBySideEnabled = true;
 
     this._listeners = new ListenerCollection();
 
@@ -223,9 +225,11 @@ export class VitalSourceContentIntegration {
     return this._htmlIntegration.contentContainer();
   }
 
-  fitSideBySide() {
-    // Not yet implemented
-    return false;
+  /**
+   * @param {SidebarLayout} layout
+   */
+  fitSideBySide(layout) {
+    return this._htmlIntegration.fitSideBySide(layout);
   }
 
   async getMetadata() {