message_fetch: Add small backfill delay.

This should help reduce the risk of hitting rate limits when users
have a very large number of messages to fetch via this mechanism.

Inline the `messages` variable that was only used in one place while
we're touching this.

web/src/message_fetch.js

@@ -38,6 +38,9 @@ const consts = {
     narrowed_view_forward_batch_size: 100,
     recent_view_fetch_more_batch_size: 1000,
     catch_up_batch_size: 1000,
+    // Delay in milliseconds after processing a catch-up request
+    // before sending the next one.
+    catch_up_backfill_delay: 150,
 };
 
 function process_result(data, opts) {
@@ -563,16 +566,19 @@ export function initialize(home_view_loaded) {
 
         // If we fall through here, we need to keep fetching more data, and
         // we'll call back to the function we're in.
-        const messages = data.messages;
-        const latest_id = messages.at(-1).id;
-
-        load_messages({
-            anchor: latest_id,
-            num_before: 0,
-            num_after: consts.catch_up_batch_size,
-            msg_list: message_lists.home,
-            cont: load_more,
-        });
+        //
+        // But we do it with a bit of delay, to reduce risk that we
+        // hit rate limits with these backfills.
+        const latest_id = data.messages.at(-1).id;
+        setTimeout(() => {
+            load_messages({
+                anchor: latest_id,
+                num_before: 0,
+                num_after: consts.catch_up_batch_size,
+                msg_list: message_lists.home,
+                cont: load_more,
+            });
+        }, consts.catch_up_backfill_delay);
     }
 
     let anchor;

web/tests/message_fetch.test.js

@@ -222,6 +222,12 @@ function forward_fill_step() {
     let fetch;
 
     self.prep = () => {
+        /* Don't wait for the timeout before recursively calling `load_messages`. */
+        const expected_delay = 150;
+        set_global("setTimeout", (f, delay) => {
+            assert.equal(delay, expected_delay);
+            f();
+        });
         fetch = config_fake_channel({
             expected_opts_data: initialize_data.forward_fill.req,
         });