model [nfc]: Comment on rest of REGISTER_COMPLETE handlers that reset state

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

> - [ ] Some reducers don't have a comment saying why they reset
>       state on `REGISTER_COMPLETE` instead of taking data from its
>       payload:
>       - For example, `flagsReducer` should point out that
>         `messagesReducer` doesn't store messages on
>         `REGISTER_COMPLETE`, and `messagesReducer` should say that
>         it doesn't do that because we fetch messages with
>         `GET /messages`.
>       - (Probably plenty of other instances.)

Fixes-partly: #5605

Author: chrisbobbe

src/caughtup/caughtUpReducer.js

@@ -32,10 +32,14 @@ export default (
   action: PerAccountApplicableAction,
 ): CaughtUpState => {
   switch (action.type) {
-    case REGISTER_COMPLETE:
     case RESET_ACCOUNT_DATA:
       return initialState;
 
+    // Reset because `caughtUp` is server-data metadata, and we're resetting
+    // the server data it's supposed to apply to: `state.narrows`.
+    case REGISTER_COMPLETE:
+      return initialState;
+
     case MESSAGE_FETCH_START: {
       // We don't want to accumulate old searches that we'll never
       // need again.

src/chat/fetchingReducer.js

@@ -71,10 +71,11 @@ export default (
       return initialState;
 
     // Reset just because `fetching` is server-data metadata, and we're
-    // resetting the server data it's supposed to apply to… But really, we
-    // should have canceled any in-progress message fetches by now; that's
-    // #5623. Still, even if there is an in-progress fetch, we probably
-    // don't want to show loading indicators for it in the UI.
+    // resetting the server data it's supposed to apply to
+    // (`state.narrows`)… But really, we should have canceled any
+    // in-progress message fetches by now; that's #5623. Still, even if
+    // there is an in-progress fetch, we probably don't want to show loading
+    // indicators for it in the UI.
     // TODO(#5623): Remove reference to #5623.
     case REGISTER_COMPLETE:
       return initialState;

src/chat/flagsReducer.js

@@ -131,10 +131,16 @@ export default (
   action: PerAccountApplicableAction,
 ): FlagsState => {
   switch (action.type) {
-    case REGISTER_COMPLETE:
     case RESET_ACCOUNT_DATA:
       return initialState;
 
+    // Reset to clear stale data. We don't initialize the
+    // messages/narrows/flags model using initial data; instead, we fetch
+    // chunks of data as needed with api.getMessages. See
+    //   https://zulip.readthedocs.io/en/latest/subsystems/events-system.html#messages
+    case REGISTER_COMPLETE:
+      return initialState;
+
     case MESSAGE_FETCH_COMPLETE:
       return processFlagsForMessages(state, action.messages);
 

src/chat/narrowsReducer.js

@@ -194,10 +194,16 @@ export default (
   action: PerAccountApplicableAction,
 ): NarrowsState => {
   switch (action.type) {
-    case REGISTER_COMPLETE:
     case RESET_ACCOUNT_DATA:
       return initialState;
 
+    // Reset to clear stale data. We don't initialize the
+    // messages/narrows/flags model using initial data; instead, we fetch
+    // chunks of data as needed with api.getMessages. See
+    //   https://zulip.readthedocs.io/en/latest/subsystems/events-system.html#messages
+    case REGISTER_COMPLETE:
+      return initialState;
+
     case MESSAGE_FETCH_START: {
       // We don't want to accumulate old searches that we'll never need again.
       if (isSearchNarrow(action.narrow)) {

src/message/messagesReducer.js

@@ -139,10 +139,16 @@ export default (
   globalState: PerAccountState,
 ): MessagesState => {
   switch (action.type) {
-    case REGISTER_COMPLETE:
     case RESET_ACCOUNT_DATA:
       return initialState;
 
+    // Reset to clear stale data. We don't initialize the
+    // messages/narrows/flags model using initial data; instead, we fetch
+    // chunks of data as needed with api.getMessages. See
+    //   https://zulip.readthedocs.io/en/latest/subsystems/events-system.html#messages
+    case REGISTER_COMPLETE:
+      return initialState;
+
     case MESSAGE_FETCH_COMPLETE:
       return state.merge(
         Immutable.Map(

src/outbox/outboxReducer.js

@@ -26,6 +26,7 @@ export default (
   action: PerAccountApplicableAction,
 ): OutboxState => {
   switch (action.type) {
+    // TODO(#3881): Figure out if we want this.
     case REGISTER_COMPLETE:
       return filterArray(state, (outbox: Outbox) => !outbox.isSent);