navReducer: Use getNavigationRoutes selector, for simplicity.

Also add a comment, with Greg's description [1] of what's going on here.

[1] zulip#4274 (comment)

src/nav/__tests__/navReducer-test.js

@@ -2,10 +2,17 @@ import deepFreeze from 'deep-freeze';
 
 import { INITIAL_FETCH_COMPLETE } from '../../actionConstants';
 import navReducer, { getStateForRoute } from '../navReducer';
+import NavigationService from '../NavigationService';
 
 describe('navReducer', () => {
   describe('INITIAL_FETCH_COMPLETE', () => {
     test('do not mutate navigation state if already at the same route', () => {
+      NavigationService.getState = jest.fn().mockReturnValue(
+        deepFreeze({
+          index: 0,
+          routes: [{ routeName: 'main' }],
+        }),
+      );
       const prevState = getStateForRoute('main');
 
       const action = deepFreeze({

src/nav/navReducer.js

@@ -2,6 +2,7 @@
 import type { NavigationAction } from 'react-navigation';
 
 import type { NavigationState, Action } from '../types';
+import { getNavigationRoutes } from './navSelectors';
 import AppNavigator from './AppNavigator';
 import { INITIAL_FETCH_COMPLETE } from '../actionConstants';
 
@@ -29,7 +30,10 @@ export const initialState = getStateForRoute('loading');
 export default (state: NavigationState = initialState, action: Action): NavigationState => {
   switch (action.type) {
     case INITIAL_FETCH_COMPLETE:
-      return state.routes[0].routeName === 'main' ? state : getStateForRoute('main');
+      // If we're anywhere in the normal UI of the app, then remain
+      // where we are. Only reset the nav state if we're elsewhere,
+      // and in that case, go to the main screen.
+      return getNavigationRoutes()[0].routeName === 'main' ? state : getStateForRoute('main');
 
     default: {
       // The `react-navigation` libdef says this only takes a NavigationAction,

src/nav/navSelectors.js

@@ -5,7 +5,7 @@ import NavigationService from './NavigationService';
 
 export const getNavState = (): NavigationState => NavigationService.getState();
 
-const getNavigationRoutes = () => getNavState().routes;
+export const getNavigationRoutes = () => getNavState().routes;
 
 const getNavigationIndex = () => getNavState().index;