pointer: Remove orig_initial_pointer hackery.

The orig_initial_pointer variable was part of the implementation for
ensuring server-initiated reloads preserve the user's selected message
and scroll position (so that they are not disruptive).  Previously,
the logic did some unnecessary contortions to ensure the two goals:

* The `pointer.js` logic knows what the server thinks the pointer is.
* The `message_fetch.js` logic knows what anchor to use to center it's
  home view fetch.

It's a lot cleaner to do this by not mutating page_params.pointer.

static/js/message_fetch.js

@@ -374,8 +374,16 @@ exports.initialize = function () {
 
     }
 
+    let anchor;
+    if (page_params.initial_pointer) {
+        // If we're doing a server-initiated reload, similar to a
+        // near: narrow query, we want to select a specific message.
+        anchor = page_params.initial_pointer;
+    } else {
+        anchor = page_params.pointer;
+    }
     exports.load_messages({
-        anchor: page_params.pointer,
+        anchor: anchor,
         num_before: consts.num_before_pointer,
         num_after: consts.num_after_pointer,
         msg_list: home_msg_list,

static/js/pointer.js

@@ -84,10 +84,6 @@ exports.fast_forward_pointer = function () {
 
 exports.initialize = function initialize() {
     exports.server_furthest_read = page_params.pointer;
-    if (page_params.orig_initial_pointer !== undefined &&
-        page_params.orig_initial_pointer > exports.server_furthest_read) {
-        exports.server_furthest_read = page_params.orig_initial_pointer;
-    }
     exports.furthest_read = exports.server_furthest_read;
 
     // We only send pointer updates when the user has been idle for a

static/js/reload.js

@@ -148,8 +148,7 @@ exports.initialize = function () {
     const pointer = parseInt(vars.pointer, 10);
 
     if (pointer) {
-        page_params.orig_initial_pointer = page_params.pointer;
-        page_params.pointer = pointer;
+        page_params.initial_pointer = pointer;
     }
     const offset = parseInt(vars.offset, 10);
     if (offset) {