nav: Update ChatScreen narrow on topic move

This covers the most important bit: when applicable, the narrow the
ChatScreen is looking at gets updated to follow the moved messages.

Left open are a couple of other things to do, related to the
compose box.  I think I'm content to close the issue without those,
and open follow-up issues for them.  Include TODO comments for them.

Fixes: #5251

Author: gnprice

src/events/doEventActionSideEffects.js

@@ -1,10 +1,19 @@
 /* @flow strict-local */
 // import { Vibration } from 'react-native';
+import { CommonActions } from '@react-navigation/native';
 
-import type { ThunkAction } from '../types';
+import type { Narrow, ThunkAction } from '../types';
 import type { EventAction } from '../actionTypes';
-import { EVENT_TYPING_START } from '../actionConstants';
+import { EVENT_TYPING_START, EVENT_UPDATE_MESSAGE } from '../actionConstants';
 import { ensureTypingStatusExpiryLoop } from '../typing/typingActions';
+import * as NavigationService from '../nav/NavigationService';
+import {
+  isStreamNarrow,
+  isTopicNarrow,
+  streamIdOfNarrow,
+  topicNarrow,
+  topicOfNarrow,
+} from '../utils/narrow';
 
 /**
  * React to actions dispatched for Zulip server events.
@@ -16,6 +25,63 @@ export default (action: EventAction): ThunkAction<Promise<void>> => async (dispa
     case EVENT_TYPING_START:
       dispatch(ensureTypingStatusExpiryLoop());
       break;
+
+    case EVENT_UPDATE_MESSAGE: {
+      // If the conversation we were looking at got moved, follow it.
+      // See #5251 for background.
+
+      const { event, move } = action;
+      const { propagate_mode } = event;
+      if (!move || !(propagate_mode === 'change_all' || propagate_mode === 'change_later')) {
+        // This edit wasn't a move, or was targeted to a specific message.
+        break;
+      }
+
+      const navState = NavigationService.getState();
+      for (const route of navState.routes) {
+        if (route.name !== 'chat') {
+          continue;
+        }
+
+        // $FlowFixMe[incompatible-type]: relying on ChatScreen route-params type
+        const narrow: Narrow = route.params?.narrow;
+        if (
+          isTopicNarrow(narrow)
+          && streamIdOfNarrow(narrow) === move.orig_stream_id
+          && topicOfNarrow(narrow) === move.orig_topic
+        ) {
+          // A ChatScreen showing the very conversation that was moved.
+
+          // Change the ChatScreen's narrow to follow the move.
+          //
+          // Web does this only if the blue box is on one of the affected
+          // messages, in case only some of the conversation was moved.
+          // We don't have a blue box, but:
+          // TODO: Ideally if the moved messages are all offscreen we'd skip this.
+          NavigationService.dispatch({
+            ...CommonActions.setParams({
+              narrow: topicNarrow(move.new_stream_id, move.new_topic),
+            }),
+            // Spreading the `setParams` action and adding `source` like
+            // this is the documented way to specify a route (rather than
+            // apply to the focused route):
+            //   https://reactnavigation.org/docs/5.x/navigation-actions#setparams
+            source: route.key,
+          });
+
+          // TODO(#5251): If compose box is open and topic was resolved, warn.
+        } else if (isStreamNarrow(narrow) && streamIdOfNarrow(narrow) === move.orig_stream_id) {
+          // A ChatScreen showing the stream that contained the moved messages.
+          //
+          // TODO(#5251): Update topic input, if it matches.  (If the stream
+          //   changed too, unclear what to do.  Possibly change the
+          //   screen's narrow, as if narrowed to the moved topic?)
+        }
+      }
+
+      break;
+    }
+
     default:
   }
 };

src/nav/navActions.js

@@ -42,6 +42,7 @@ export const resetToMainTabs = (): GenericNavigationAction =>
 
 /** Only call this via `doNarrow`.  See there for details. */
 export const navigateToChat = (narrow: Narrow): GenericNavigationAction =>
+  // This route name 'chat' appears in one more place than usual: doEventActionSideEffects.js .
   StackActions.push('chat', { narrow, editMessage: null });
 
 export const replaceWithChat = (narrow: Narrow): GenericNavigationAction =>