Simplify stream sidebar narrows.

This fix simplifies how we re-render topic lists when we
re-narrow or zoom out from a topic list.

    * The topic_list.zoom_out() no longer gets called as
      part of re-narrowing, and we eliminate the clear_topics
      option.

    * For all situations where we narrow to a filter that does
      not have a topic, we simply call the new function
      clear_topics().

    * The stream_list code no longer calls remove_expanded_topics()
      in cases where the new narrow has a topic.  This allows us
      to optimize away scroll/flicker churn a little more easily.

As part of this, we rename maybe_activate_stream_item() to
update_stream_sidebar_for_narrow(), since the function clears
stuff as well as turning stuff on.

frontend_tests/node_tests/stream_list.js

@@ -239,9 +239,11 @@ function initialize_stream_data() {
     });
 
     topic_list.set_click_handlers = noop;
-    topic_list.is_zoomed = return_false;
+    topic_list.close = noop;
     topic_list.remove_expanded_topics = noop;
     topic_list.rebuild = noop;
+    topic_list.active_stream_id = noop;
+    stream_list.show_all_streams = noop;
     stream_list.scroll_element_into_container = noop;
 
     var scrollbar_updated = false;

static/js/stream_list.js

@@ -180,7 +180,7 @@ function zoom_in() {
 
 function zoom_out(options) {
     popovers.hide_all();
-    topic_list.zoom_out(options);
+    topic_list.zoom_out();
 
     if (options.stream_li) {
         exports.scroll_stream_into_view(options.stream_li);
@@ -201,14 +201,6 @@ exports.show_all_streams = function () {
     $("#stream_filters li.narrow-filter").show();
 };
 
-function reset_to_unnarrowed(narrowed_within_same_stream) {
-    if (topic_list.is_zoomed() && narrowed_within_same_stream !== true) {
-        zoom_out({clear_topics: true});
-    } else {
-        topic_list.remove_expanded_topics();
-    }
-}
-
 exports.set_in_home_view = function (stream_id, in_home) {
     var li = exports.get_stream_li(stream_id);
     if (!li) {
@@ -317,7 +309,7 @@ exports.update_streams_sidebar = function () {
 
     var filter = narrow_state.filter();
 
-    exports.maybe_activate_stream_item(filter);
+    exports.update_stream_sidebar_for_narrow(filter);
 };
 
 exports.update_dom_with_unread_counts = function (counts) {
@@ -379,6 +371,11 @@ exports.refresh_pinned_or_unpinned_stream = function (sub) {
     }
 };
 
+function clear_topics() {
+    topic_list.close();
+    exports.show_all_streams();
+}
+
 exports.get_sidebar_stream_topic_info  = function (filter) {
     var result = {
         stream_id: undefined,
@@ -409,12 +406,13 @@ exports.get_sidebar_stream_topic_info  = function (filter) {
     return result;
 };
 
-exports.maybe_activate_stream_item = function (filter) {
+exports.update_stream_sidebar_for_narrow = function (filter) {
     var info = exports.get_sidebar_stream_topic_info(filter);
 
     var stream_id = info.stream_id;
 
     if (!stream_id) {
+        clear_topics();
         return;
     }
 
@@ -427,6 +425,7 @@ exports.maybe_activate_stream_item = function (filter) {
         // evidence that this assumption breaks down for some users,
         // but we are not clear why it happens.
         blueslip.error('No stream_li for subscribed stream ' + stream_id);
+        clear_topics();
         return;
     }
 
@@ -434,6 +433,10 @@ exports.maybe_activate_stream_item = function (filter) {
         stream_li.addClass('active-filter');
     }
 
+    if (stream_id !== topic_list.active_stream_id()) {
+        clear_topics();
+    }
+
     topic_list.rebuild(stream_li, stream_id);
 
     return stream_li;
@@ -489,9 +492,7 @@ exports.initialize = function () {
     $(document).on('narrow_activated.zulip', function (event) {
         exports.update_top_left_corner_for_narrow(event.filter);
 
-        reset_to_unnarrowed(narrow_state.stream() === zoomed_stream);
-
-        var stream_li = exports.maybe_activate_stream_item(event.filter);
+        var stream_li = exports.update_stream_sidebar_for_narrow(event.filter);
         if (stream_li) {
             exports.scroll_stream_into_view(stream_li);
         }
@@ -501,7 +502,7 @@ exports.initialize = function () {
 
     $(document).on('narrow_deactivated.zulip', function () {
         deselect_top_left_corner_items();
-        reset_to_unnarrowed();
+        clear_topics();
         pm_list.close();
 
         var filter_li = exports.get_global_filter_li('home');

static/js/topic_list.js

@@ -13,9 +13,20 @@ exports.remove_expanded_topics = function () {
 
     if (active_widget) {
         active_widget.remove();
+        active_widget = undefined;
     }
 };
 
+exports.close = function () {
+    zoomed = false;
+    exports.remove_expanded_topics();
+};
+
+exports.zoom_out = function () {
+    zoomed = false;
+    exports.rebuild(active_widget.get_parent(), active_widget.get_stream_id());
+};
+
 function update_unread_count(unread_count_elem, count) {
     // unread_count_elem is a jquery element...we expect DOM
     // to look like this:
@@ -198,19 +209,6 @@ exports.zoom_in = function () {
     }
 };
 
-exports.zoom_out = function (options) {
-    zoomed = false;
-    if (options && options.clear_topics) {
-        exports.remove_expanded_topics();
-    } else {
-        exports.rebuild(active_widget.get_parent(), active_widget.get_stream_id());
-    }
-};
-
-exports.is_zoomed = function () {
-    return zoomed;
-};
-
 exports.set_click_handlers = function (callbacks) {
     $('#stream_filters').on('click', '.show-more-topics', function (e) {
         callbacks.zoom_in();
@@ -221,7 +219,6 @@ exports.set_click_handlers = function (callbacks) {
 
     $('.show-all-streams').on('click', function (e) {
         callbacks.zoom_out({
-            clear_topics: false,
             stream_li: active_widget.get_parent(),
         });