Move message_viewport_info() from ui.js to viewport.js

The message_viewport_info() function encapsulates our logic
around the compose box and other elements blocking the viewport,
so viewport.js seems like a more logical home for it.  It also
makes ui.js, one of our largest modules, a little bit smaller.

(imported from commit 7838668b28175e161b87a6d7a8124b73012f0ff3)

zephyr/static/js/message_list.js

@@ -442,7 +442,7 @@ MessageList.prototype = {
         }
 
         var selected_row_offset = selected_row.offset().top;
-        var info = ui.message_viewport_info();
+        var info = viewport.message_viewport_info();
         var available_space_for_scroll = selected_row_offset - info.visible_top;
 
         // Don't scroll if we can't move the pointer up.

zephyr/static/js/ui.js

@@ -32,38 +32,11 @@ exports.focus_on = function (field_id) {
     $("#" + field_id).focus();
 };
 
-exports.message_viewport_info = function () {
-    // Return a structure that tells us details of the viewport
-    // accounting for fixed elements like the top navbar.
-    //
-    // The message_header is NOT considered to be part of the visible
-    // message pane, which should make sense for callers, who will
-    // generally be concerned about whether actual message content is
-    // visible.
-
-    var res = {};
-
-    var element_just_above_us = $("#tab_bar_underpadding");
-
-    res.visible_top =
-        element_just_above_us.offset().top
-        + element_just_above_us.height()
-        + $(".message_header").height();
-
-    var element_just_below_us = $("#compose");
-
-    res.visible_height =
-        element_just_below_us.offset().top
-        - res.visible_top;
-
-    return res;
-};
-
 function amount_to_paginate() {
     // Some day we might have separate versions of this function
     // for Page Up vs. Page Down, but for now it's the same
     // strategy in either direction.
-    var info = exports.message_viewport_info();
+    var info = viewport.message_viewport_info();
     var page_size = info.visible_height;
 
     // We don't want to page up a full page, because Humbug users

zephyr/static/js/viewport.js

@@ -10,6 +10,33 @@ exports.at_top = function () {
     return (jwindow.scrollTop() <= 0);
 };
 
+exports.message_viewport_info = function () {
+    // Return a structure that tells us details of the viewport
+    // accounting for fixed elements like the top navbar.
+    //
+    // The message_header is NOT considered to be part of the visible
+    // message pane, which should make sense for callers, who will
+    // generally be concerned about whether actual message content is
+    // visible.
+
+    var res = {};
+
+    var element_just_above_us = $("#tab_bar_underpadding");
+
+    res.visible_top =
+        element_just_above_us.offset().top
+        + element_just_above_us.height()
+        + $(".message_header").height();
+
+    var element_just_below_us = $("#compose");
+
+    res.visible_height =
+        element_just_below_us.offset().top
+        - res.visible_top;
+
+    return res;
+};
+
 exports.at_bottom = function () {
     // outerHeight(true): Include margin
     var bottom = jwindow.scrollTop() + jwindow.height();
@@ -25,7 +52,7 @@ exports.at_bottom = function () {
 exports.set_message_position = function (message_top, message_height, viewport_info, ratio) {
     // message_top = offset of the top of a message that you are positioning
     // message_height = height of the message that you are positioning
-    // viewport_info = result of calling ui.message_viewport_info
+    // viewport_info = result of calling viewport.message_viewport_info
     // ratio = fraction indicating how far down the screen the msg should be
 
     var how_far_down_in_visible_page = viewport_info.visible_height * ratio;

zephyr/static/js/zephyr.js

@@ -75,7 +75,7 @@ function within_viewport(row_offset, row_height) {
     // part of the viewport.
     var message_top = row_offset.top;
     var message_bottom  = message_top + row_height;
-    var info = ui.message_viewport_info();
+    var info = viewport.message_viewport_info();
     var viewport_top = info.visible_top;
     var viewport_bottom = viewport_top + info.visible_height;
     return (message_top > viewport_top) && (message_bottom < viewport_bottom);
@@ -94,7 +94,7 @@ function keep_pointer_in_view() {
     if (next_row.length === 0)
         return;
 
-    var info = ui.message_viewport_info();
+    var info = viewport.message_viewport_info();
     var top_threshold = info.visible_top + (1/10 * info.visible_height);
     var bottom_threshold = info.visible_top + (9/10 * info.visible_height);
 
@@ -130,7 +130,7 @@ function recenter_view(message, from_scroll) {
     // the 1/2 marks. If the pointer is too low, move it to the 1/7 mark.
     // See keep_pointer_in_view() for related logic to keep the pointer onscreen.
 
-    var viewport_info = ui.message_viewport_info();
+    var viewport_info = viewport.message_viewport_info();
     var top_threshold = viewport_info.visible_top;
 
     var bottom_threshold = viewport_info.visible_top + viewport_info.visible_height;
@@ -342,7 +342,7 @@ function process_visible_unread_messages(update_cursor) {
     }
 
     var selected = current_msg_list.selected_message();
-    var vp = ui.message_viewport_info();
+    var vp = viewport.message_viewport_info();
     var top = vp.visible_top;
     var height = vp.visible_height;