Improve up button behavior

- Closes #614

app/src/main/java/org/schabi/newpipe/MainActivity.java

@@ -204,9 +204,37 @@ public class MainActivity extends AppCompatActivity implements HistoryListener {
         } else super.onBackPressed();
     }
 
+    /**
+     * Implement the following diagram behavior for the up button:
+     * <pre>
+     *              +---------------+
+     *              |  Main Screen  +----+
+     *              +-------+-------+    |
+     *                      |            |
+     *                      ▲ Up         | Search Button
+     *                      |            |
+     *                 +----+-----+      |
+     *    +------------+  Search  |◄-----+
+     *    |            +----+-----+
+     *    |   Open          |
+     *    |  something      ▲ Up
+     *    |                 |
+     *    |    +------------+-------------+
+     *    |    |                          |
+     *    |    |  Video    <->  Channel   |
+     *    +---►|  Channel  <->  Playlist  |
+     *         |  Video    <->  ....      |
+     *         |                          |
+     *         +--------------------------+
+     * </pre>
+     */
     private void onHomeButtonPressed() {
+        // If search fragment wasn't found in the backstack...
+        if (!NavigationHelper.tryGotoSearchFragment(getSupportFragmentManager())) {
+            // ...go to the main fragment
             NavigationHelper.gotoMainFragment(getSupportFragmentManager());
         }
+    }
 
     /*//////////////////////////////////////////////////////////////////////////
     // Menu
@@ -239,11 +267,6 @@ public class MainActivity extends AppCompatActivity implements HistoryListener {
         return true;
     }
 
-    @Override
-    public boolean onPrepareOptionsMenu(Menu menu) {
-        return super.onPrepareOptionsMenu(menu);
-    }
-
     @Override
     public boolean onOptionsItemSelected(MenuItem item) {
         if (DEBUG) Log.d(TAG, "onOptionsItemSelected() called with: item = [" + item + "]");

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -51,6 +51,7 @@ import java.util.ArrayList;
 @SuppressWarnings({"unused", "WeakerAccess"})
 public class NavigationHelper {
     public static final String MAIN_FRAGMENT_TAG = "main_fragment_tag";
+    public static final String SEARCH_FRAGMENT_TAG = "search_fragment_tag";
 
     /*//////////////////////////////////////////////////////////////////////////
     // Players
@@ -244,11 +245,21 @@ public class NavigationHelper {
                 .commit();
     }
 
+    public static boolean tryGotoSearchFragment(FragmentManager fragmentManager) {
+        if (MainActivity.DEBUG) {
+            for (int i = 0; i < fragmentManager.getBackStackEntryCount(); i++) {
+                Log.d("NavigationHelper", "tryGoToSearchFragment() [" + i + "] = [" + fragmentManager.getBackStackEntryAt(i) + "]");
+            }
+        }
+
+        return fragmentManager.popBackStackImmediate(SEARCH_FRAGMENT_TAG, 0);
+    }
+
     public static void openSearchFragment(FragmentManager fragmentManager, int serviceId, String query) {
         fragmentManager.beginTransaction()
                 .setCustomAnimations(R.animator.custom_fade_in, R.animator.custom_fade_out, R.animator.custom_fade_in, R.animator.custom_fade_out)
                 .replace(R.id.fragment_holder, SearchFragment.getInstance(serviceId, query))
-                .addToBackStack(null)
+                .addToBackStack(SEARCH_FRAGMENT_TAG)
                 .commit();
     }