[Navigation.Item] Fix onClick not being called on small screens

UNRELEASED.md

@@ -8,6 +8,7 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 
 ### Bug fixes
 
+- Fixed `Navigation.Item` not calling `onClick` on small screens when `onNavigationDismiss` is undefined ([#603](https://github.com/Shopify/polaris-react/pull/603))
 - Fixed `Autocomplete` empty state example Markdown not parsing correctly ([#592](https://github.com/Shopify/polaris-react/pull/592))
 
 ### Documentation

src/components/Navigation/components/Item/Item.tsx

@@ -294,7 +294,7 @@ export class BaseItem extends React.Component<CombinedProps, State> {
         return;
       }
 
-      if (onClick && !navigationBarCollapsed().matches) {
+      if (onClick) {
         onClick();
       }
     };

src/components/Navigation/components/Item/tests/Item.test.tsx

@@ -358,6 +358,55 @@ describe('<Nav.Item />', () => {
         .exists(),
     ).toBe(true);
   });
+
+  describe('small screens', () => {
+    let matchMedia: jest.SpyInstance;
+    beforeEach(() => {
+      matchMedia = jest.spyOn(window, 'matchMedia');
+      matchMedia.mockImplementation(() => {
+        return {
+          matches: true,
+          addListener() {},
+          removeListener() {},
+        };
+      });
+    });
+
+    afterEach(() => {
+      matchMedia.mockRestore();
+    });
+
+    describe('onClick', () => {
+      it('will fire once on small screens when onNavigationDismiss is defined', () => {
+        const item = mountWithAppProvider(
+          <Item label="some label" disabled={false} onClick={jest.fn()} />,
+          {
+            context: {
+              location: 'bar',
+              onNavigationDismiss: noop,
+            },
+          },
+        );
+
+        item.find('button').simulate('click');
+        expect(item.prop('onClick')).toHaveBeenCalledTimes(1);
+      });
+
+      it('will fire once on small screens when onNavigationDismiss is undefined', () => {
+        const item = mountWithAppProvider(
+          <Item label="some label" disabled={false} onClick={jest.fn()} />,
+          {
+            context: {
+              location: 'bar',
+            },
+          },
+        );
+
+        item.find('button').simulate('click');
+        expect(item.prop('onClick')).toHaveBeenCalledTimes(1);
+      });
+    });
+  });
 });
 
 function itemForLocation(location: string, overrides: Partial<ItemProps> = {}) {