Do not change to backdrops tab after adding a backdrop.

Expand on the backdrop integration test to make sure this is documented/tested behavior

src/containers/backdrop-library.jsx

@@ -5,11 +5,6 @@ import {defineMessages, injectIntl, intlShape} from 'react-intl';
 import {connect} from 'react-redux';
 import VM from 'scratch-vm';
 
-import {
-    activateTab,
-    COSTUMES_TAB_INDEX
-} from '../reducers/editor-tab';
-
 import analytics from '../lib/analytics';
 import backdropLibraryContent from '../lib/libraries/backdrops.json';
 import backdropTags from '../lib/libraries/backdrop-tags';
@@ -40,7 +35,6 @@ class BackdropLibrary extends React.Component {
             skinId: null
         };
         this.props.vm.setEditingTarget(this.props.stageID);
-        this.props.onActivateTab(COSTUMES_TAB_INDEX);
         this.props.vm.addBackdrop(item.md5, vmBackdrop);
         analytics.event({
             category: 'library',
@@ -64,7 +58,6 @@ class BackdropLibrary extends React.Component {
 
 BackdropLibrary.propTypes = {
     intl: intlShape.isRequired,
-    onActivateTab: PropTypes.func.isRequired,
     onRequestClose: PropTypes.func,
     stageID: PropTypes.string.isRequired,
     vm: PropTypes.instanceOf(VM).isRequired
@@ -74,9 +67,7 @@ const mapStateToProps = state => ({
     stageID: state.scratchGui.targets.stage.id
 });
 
-const mapDispatchToProps = dispatch => ({
-    onActivateTab: tab => dispatch(activateTab(tab))
-});
+const mapDispatchToProps = () => ({});
 
 export default injectIntl(connect(
     mapStateToProps,

test/integration/backdrops.test.js

+import path from 'path';
+import SeleniumHelper from '../helpers/selenium-helper';
+
+const {
+    clickText,
+    clickXpath,
+    findByXpath,
+    getDriver,
+    getLogs,
+    loadUri,
+    scope
+} = new SeleniumHelper();
+
+const uri = path.resolve(__dirname, '../../build/index.html');
+
+let driver;
+
+describe('Working with backdrops', () => {
+    beforeAll(() => {
+        driver = getDriver();
+    });
+
+    afterAll(async () => {
+        await driver.quit();
+    });
+
+    test('Adding a backdrop from the library', async () => {
+        await loadUri(uri);
+        await clickXpath('//button[@title="Try It"]');
+
+        // Start on the sounds tab of sprite1 to test switching behavior
+        await clickText('Sounds');
+
+        // Add a backdrop without selecting the stage first to test switching
+        await clickXpath('//button[@aria-label="Choose a Backdrop"]');
+        const el = await findByXpath("//input[@placeholder='Search']");
+        await el.sendKeys('blue');
+        await clickText('Blue Sky'); // Adds the backdrop
+
+        // Make sure the stage is selected and the sound tab remains selected.
+        // This is different from Scratch2 which selected backdrop tab automatically
+        // See issue #3500
+        await clickText('pop', scope.soundsTab);
+
+        // Make sure the backdrop was actually added by going to the backdrops tab
+        await clickText('Backdrops');
+        await clickText('Blue Sky', scope.costumesTab);
+
+        const logs = await getLogs();
+        await expect(logs).toEqual([]);
+    });
+});

test/integration/costumes.test.js

@@ -80,17 +80,6 @@ describe('Working with costumes', () => {
         await expect(logs).toEqual([]);
     });
 
-    test('Adding a backdrop', async () => {
-        await loadUri(uri);
-        await clickXpath('//button[@title="Try It"]');
-        await clickXpath('//button[@aria-label="Choose a Backdrop"]');
-        const el = await findByXpath("//input[@placeholder='Search']");
-        await el.sendKeys('blue');
-        await clickText('Blue Sky'); // Should close the modal
-        const logs = await getLogs();
-        await expect(logs).toEqual([]);
-    });
-
     test('Converting bitmap/vector in paint editor', async () => {
         await loadUri(uri);
         await clickXpath('//button[@title="Try It"]');