pointer: Move scroll suppression to message_scroll.js.

This is clearly a better home for it, since message_scroll.js is the
only place that reads it, and also lets us provide a clearer name for
the functionality.

static/js/message_scroll.js

@@ -1,5 +1,13 @@
 let actively_scrolling = false;
 
+// Tracks whether the next scroll that will complete is initiated by
+// code, not the user, and thus should avoid moving the selected
+// message.
+let update_selection_on_next_scroll = true;
+exports.suppress_selection_update_on_next_scroll = function () {
+    update_selection_on_next_scroll = false;
+};
+
 let loading_older_messages_indicator_showing = false;
 let loading_newer_messages_indicator_showing = false;
 exports.show_loading_older = function () {
@@ -111,10 +119,10 @@ exports.scroll_finished = function () {
         return;
     }
 
-    if (!pointer.suppress_scroll_pointer_update) {
+    if (update_selection_on_next_scroll) {
         message_viewport.keep_pointer_in_view();
     } else {
-        pointer.set_suppress_scroll_pointer_update(false);
+        update_selection_on_next_scroll = true;
     }
 
     floating_recipient_bar.update();

static/js/message_viewport.js

@@ -123,7 +123,7 @@ exports.set_message_position = function (message_top, message_height, viewport_i
         message_top
         - message_offset;
 
-    pointer.set_suppress_scroll_pointer_update(true); // Gets set to false in the scroll handler.
+    message_scroll.suppress_selection_update_on_next_scroll();
     exports.scrollTop(new_scroll_top);
 };
 
@@ -280,7 +280,7 @@ exports.is_narrow = function () {
 };
 
 exports.system_initiated_animate_scroll = function (scroll_amount) {
-    pointer.set_suppress_scroll_pointer_update(true); // Gets set to false in the scroll handler.
+    message_scroll.suppress_selection_update_on_next_scroll();
     const viewport_offset = exports.scrollTop();
     in_stoppable_autoscroll = true;
     exports.message_pane.animate({
@@ -292,7 +292,7 @@ exports.system_initiated_animate_scroll = function (scroll_amount) {
 };
 
 exports.user_initiated_animate_scroll = function (scroll_amount) {
-    pointer.set_suppress_scroll_pointer_update(true); // Gets set to false in the scroll handler.
+    message_scroll.suppress_selection_update_on_next_scroll();
     in_stoppable_autoscroll = false; // defensive
 
     const viewport_offset = exports.scrollTop();

static/js/pointer.js

@@ -6,13 +6,6 @@ exports.set_recenter_pointer_on_display = function (value) {
     exports.recenter_pointer_on_display = value;
 };
 
-// Toggles re-centering the pointer in the window
-// when All Messages is next clicked by the user
-exports.suppress_scroll_pointer_update = false;
-exports.set_suppress_scroll_pointer_update = function (value) {
-    exports.suppress_scroll_pointer_update = value;
-};
-
 exports.initialize = function initialize() {
     $(document).on('message_selected.zulip', function (event) {
         if (event.id === -1) {

static/js/stream_muting.js

@@ -35,7 +35,7 @@ exports.update_is_muted = function (sub, value) {
         // make sure the pointer is still visible. We don't want the auto-scroll handler to move
         // our pointer to the old scroll location before we have a chance to update it.
         pointer.set_recenter_pointer_on_display(true);
-        pointer.set_suppress_scroll_pointer_update(true);
+        message_scroll.suppress_selection_update_on_next_scroll();
 
         if (!home_msg_list.empty()) {
             message_util.do_unread_count_updates(home_msg_list.all_messages());