BackNavigationHandler [nfc]: Comment on our use of navigateBack.

BackNavigationHandler is no longer necessary to support
`react-navigation-redux-helpers`, which we recently removed, but it
does this other important thing that's easy to forget about. So,
make a note.

As Greg says [1], we may want to change the behavior as part of a UI
redesign; it's not particularly intuitive.

[1] #4278 (comment)

Author: chrisbobbe

src/nav/BackNavigationHandler.js

@@ -22,7 +22,12 @@ export default class BackNavigationHandler extends PureComponent<Props> {
   handleBackButtonPress = () => {
     const canGoBack = NavigationService.getState().index > 0;
     if (canGoBack) {
-      NavigationService.dispatch(navigateBack());
+      NavigationService.dispatch(
+        // Our custom "go-back" action. If this is changed to align
+        // with React Navigation's natural "go-back" behavior, this
+        // whole component can go away.
+        navigateBack(),
+      );
     }
     return canGoBack;
   };