fix(top-app-bar): Move scroll target initialization; improve test (#4106)

Author: kfranqueiro

packages/mdc-top-app-bar/index.js

@@ -63,6 +63,8 @@ class MDCTopAppBar extends MDCComponent {
       ripple.unbounded = true;
       return ripple;
     });
+
+    this.scrollTarget_ = window;
   }
 
   destroy() {
@@ -119,8 +121,6 @@ class MDCTopAppBar extends MDCComponent {
     })
     );
 
-    this.scrollTarget_ = window;
-
     /** @type {!MDCTopAppBarBaseFoundation} */
     let foundation;
     if (this.root_.classList.contains(cssClasses.SHORT_CLASS)) {

test/unit/mdc-top-app-bar/mdc-top-app-bar.test.js

@@ -62,7 +62,6 @@ function getFixture(removeIcon) {
       </header>
       <main class="mdc-top-app-bar-fixed-adjust">
       </main>
-      <div class="content">Content</div>
     </div>
   `;
 
@@ -291,10 +290,10 @@ test('adapter#getViewportScrollY returns scroll distance', () => {
 });
 
 test('adapter#getViewportScrollY returns scroll distance when scrollTarget_ is not window', () => {
-  const {component, fixture} = setupTest();
-  const content = fixture.querySelector('.content');
-  component.scrollTarget_ = content;
-  assert.equal(component.getDefaultFoundation().adapter_.getViewportScrollY(), content.scrollTop);
+  const {component} = setupTest();
+  const mockContent = {addEventListener: () => {}, scrollTop: 20};
+  component.setScrollTarget(mockContent);
+  assert.equal(component.getDefaultFoundation().adapter_.getViewportScrollY(), mockContent.scrollTop);
 });
 
 test('adapter#getTotalActionItems returns the number of action items on the opposite side of the menu', () => {