Deactive side-by-side mode in VitalSource PDF books when client is unloaded

For EPUB books, this is handled by VitalSourceContentIntegration delegating to
HTMLIntegration.

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

@@ -275,9 +275,12 @@ describe('annotator/integrations/vitalsource', () => {
   }
 
   describe('VitalSourceContentIntegration', () => {
+    // List of active integrations.
     let integrations;
+
     let fakeBookElement;
 
+    /** Create a new integration and add it to the active list. */
     function createIntegration() {
       const integration = new VitalSourceContentIntegration(document.body, {
         bookElement: fakeBookElement,
@@ -286,6 +289,16 @@ describe('annotator/integrations/vitalsource', () => {
       return integration;
     }
 
+    /** Destroy an integration and remove it from the active list. */
+    function destroyIntegration(integration) {
+      const idx = integrations.indexOf(integration);
+      if (idx === -1) {
+        throw new Error('Integration is not in list of active integrations');
+      }
+      integrations.splice(idx, 1);
+      integration.destroy();
+    }
+
     beforeEach(() => {
       integrations = [];
       fakeBookElement = new FakeMosaicBookElement();
@@ -685,6 +698,18 @@ describe('annotator/integrations/vitalsource', () => {
         assert.calledOnce(fakeImageTextLayer.destroy);
       });
 
+      it('disables side-by-side mode when destroyed', () => {
+        createPageImageAndData();
+        const integration = createIntegration();
+
+        integration.fitSideBySide({ expanded: true, width: 100 });
+        assert.isTrue(integration.sideBySideActive());
+
+        destroyIntegration(integration);
+
+        assert.isFalse(integration.sideBySideActive());
+      });
+
       context('when side-by-side mode is toggled', () => {
         it('resizes page image', () => {
           createPageImageAndData();

src/annotator/integrations/vitalsource.ts

@@ -308,7 +308,19 @@ export class VitalSourceContentIntegration
   }
 
   destroy() {
-    this._textLayer?.destroy();
+    if (this._textLayer) {
+      this._textLayer.destroy();
+
+      // Turn off side-by-side for PDF books. For EPUBs this is handled by
+      // `this._htmlIntegration.destroy()`.
+      this.fitSideBySide({
+        // Dummy layout. Setting `expanded: false` disables side-by-side mode.
+        expanded: false,
+        height: window.innerHeight,
+        width: 0,
+        toolbarWidth: 0,
+      });
+    }
     this._listeners.removeAll();
     this._htmlIntegration.destroy();
   }