On modal mount, push history if current history state doesn't match this modal's id

src/containers/modal.jsx

@@ -19,7 +19,7 @@ class Modal extends React.Component {
     componentDidMount () {
         // Add a history event only if it's not currently for our modal. This
         // avoids polluting the history with many entries. We only need one.
-        this.pushHistory(this.id, history.state === null);
+        this.pushHistory(this.id, (history.state === null || history.state !== this.id));
     }
     componentWillUnmount () {
         this.removeEventListeners();

test/helpers/selenium-helper.js

@@ -14,6 +14,7 @@ class SeleniumHelper {
             'clickText',
             'clickButton',
             'clickXpath',
+            'elementIsVisible',
             'findByText',
             'findByXpath',
             'getDriver',
@@ -24,6 +25,10 @@ class SeleniumHelper {
         ]);
     }
 
+    elementIsVisible (element) {
+        return this.driver.wait(until.elementIsVisible(element));
+    }
+
     get scope () {
         // List of useful xpath scopes for finding elements
         return {

test/integration/sprites.test.js

@@ -4,6 +4,7 @@ import SeleniumHelper from '../helpers/selenium-helper';
 const {
     clickText,
     clickXpath,
+    elementIsVisible,
     findByText,
     findByXpath,
     getDriver,
@@ -124,4 +125,27 @@ describe('Working with sprites', () => {
         const logs = await getLogs();
         await expect(logs).toEqual([]);
     });
+
+    test('Use browser back button to close library', async () => {
+        await driver.get('https://www.google.com');
+        await loadUri(uri);
+        await clickXpath('//button[@title="Try It"]');
+        await clickText('Costumes');
+        await clickXpath('//button[@aria-label="Choose a Sprite"]');
+        const abbyElement = await findByText('Abby'); // Should show editor for new costume
+        await elementIsVisible(abbyElement);
+        await driver.navigate().back();
+        try {
+            // should throw error because library is no longer visible
+            await elementIsVisible(abbyElement);
+            throw 'ShouldNotGetHere'; // eslint-disable-line no-throw-literal
+        } catch (e) {
+            expect(e.constructor.name).toEqual('StaleElementReferenceError');
+        }
+        const costumesElement = await findByText('Costumes'); // Should show editor for new costume
+        await elementIsVisible(costumesElement);
+        const logs = await getLogs();
+        await expect(logs).toEqual([]);
+    });
+
 });