model: Make REGISTER_COMPLETE reset some state we forgot about

This addresses the second item in the list at #5605:

> - [ ] `REGISTER_COMPLETE` should reset or replace all server data
>       and server-data metadata, but it isn't in some cases:
>       - server data: `topics`, `typing` (should reset)
>       - server-data metadata: `fetching` (should reset)

Although, I've left a comment on `fetchingReducer`, where doing so
felt a bit funny because morally it shouldn't be necessary.

Fixes-partly: #5605

src/chat/__tests__/fetchingReducer-test.js

@@ -14,6 +14,15 @@ import { MESSAGE_FETCH_START, MESSAGE_FETCH_ERROR } from '../../actionConstants'
 import { DEFAULT_FETCHING } from '../fetchingSelectors';
 
 describe('fetchingReducer', () => {
+  test('REGISTER_COMPLETE', () => {
+    expect(
+      [
+        { type: MESSAGE_FETCH_START, narrow: HOME_NARROW, numBefore: 10, numAfter: 10 },
+        eg.action.register_complete,
+      ].reduce(fetchingReducer, eg.baseReduxState.fetching),
+    ).toEqual(eg.baseReduxState.fetching);
+  });
+
   describe('MESSAGE_FETCH_START', () => {
     test('if messages are fetched before or after the corresponding flag is set', () => {
       const prevState = deepFreeze({ [HOME_NARROW_STR]: { older: false, newer: false } });

src/chat/__tests__/narrowsReducer-test.js

@@ -31,6 +31,15 @@ describe('narrowsReducer', () => {
   const egTopic = eg.streamMessage().subject;
   const topicNarrowStr = keyFromNarrow(topicNarrow(eg.stream.stream_id, egTopic));
 
+  test('REGISTER_COMPLETE', () => {
+    expect(
+      [
+        { ...eg.action.message_fetch_complete, messages: [eg.streamMessage()] },
+        eg.action.register_complete,
+      ].reduce(narrowsReducer, eg.baseReduxState.narrows),
+    ).toEqual(eg.baseReduxState.narrows);
+  });
+
   describe('EVENT_NEW_MESSAGE', () => {
     test('if not caught up in narrow, do not add message in home narrow', () => {
       const initialState = Immutable.Map({ [HOME_NARROW_STR]: [1, 2] });

src/chat/fetchingReducer.js

@@ -5,6 +5,7 @@ import {
   MESSAGE_FETCH_ERROR,
   MESSAGE_FETCH_COMPLETE,
   RESET_ACCOUNT_DATA,
+  REGISTER_COMPLETE,
 } from '../actionConstants';
 import { NULL_OBJECT } from '../nullObjects';
 import { DEFAULT_FETCHING } from './fetchingSelectors';
@@ -67,6 +68,12 @@ export default (
 ): FetchingState => {
   switch (action.type) {
     case RESET_ACCOUNT_DATA:
+
+    // Reset just because `fetching` is server-data metadata, and we're
+    // resetting the server data it's supposed to apply to… but really, if
+    // there's an in-progress message fetch now, it's a bug, perhaps #5609.
+    // TODO(#5609): Remove "perhaps #5609" when that specific bug is fixed.
+    case REGISTER_COMPLETE: // eslint-disable-line no-fallthrough
       return initialState;
 
     case MESSAGE_FETCH_START:

src/topics/__tests__/topicsReducer-test.js

@@ -8,6 +8,15 @@ import { INIT_TOPICS } from '../../actionConstants';
 import { NULL_OBJECT } from '../../nullObjects';
 
 describe('topicsReducer', () => {
+  describe('REGISTER_COMPLETE', () => {
+    test('resets state to initial state', () => {
+      const prevState = deepFreeze({ [eg.stream.stream_id]: [{ max_id: 1, name: 'some topic' }] });
+      expect(topicsReducer(prevState, eg.action.register_complete)).toEqual(
+        eg.baseReduxState.topics,
+      );
+    });
+  });
+
   describe('RESET_ACCOUNT_DATA', () => {
     test('resets state to initial state', () => {
       const prevState = deepFreeze({ [eg.stream.stream_id]: [{ max_id: 1, name: 'some topic' }] });

src/topics/topicsReducer.js

@@ -1,6 +1,11 @@
 /* @flow strict-local */
 import type { TopicsState, PerAccountApplicableAction } from '../types';
-import { INIT_TOPICS, EVENT_NEW_MESSAGE, RESET_ACCOUNT_DATA } from '../actionConstants';
+import {
+  INIT_TOPICS,
+  EVENT_NEW_MESSAGE,
+  REGISTER_COMPLETE,
+  RESET_ACCOUNT_DATA,
+} from '../actionConstants';
 import { NULL_OBJECT } from '../nullObjects';
 import { replaceItemInArray } from '../utils/immutability';
 
@@ -42,6 +47,9 @@ export default (
 ): TopicsState => {
   switch (action.type) {
     case RESET_ACCOUNT_DATA:
+
+    // Reset to clear stale data; payload has no initial data for this model
+    case REGISTER_COMPLETE: // eslint-disable-line no-fallthrough
       return initialState;
 
     case INIT_TOPICS:

src/typing/typingReducer.js

@@ -6,6 +6,7 @@ import {
   CLEAR_TYPING,
   DEAD_QUEUE,
   RESET_ACCOUNT_DATA,
+  REGISTER_COMPLETE,
 } from '../actionConstants';
 import { pmTypingKeyFromRecipients } from '../utils/recipient';
 import { NULL_OBJECT } from '../nullObjects';
@@ -97,6 +98,9 @@ export default (
 
     case DEAD_QUEUE:
     case RESET_ACCOUNT_DATA:
+
+    // Reset to clear stale data; payload has no initial data for this model
+    case REGISTER_COMPLETE: // eslint-disable-line no-fallthrough
       return initialState;
 
     default: