list_render: Extract get_list_scrolling_container().

We put this in `scroll_util` to make it more likely
we will eventually unify this with other scrolling
logic.  (A big piece to move is ui.get_scroll_element,
but that's for another PR.)

And then the other tactical advantage is that we get
100% line coverage on it.

I changed the warning to an error, since I don't
think we ever expect scrolling at the `body` level,
and I don't bother with the preview node.

frontend_tests/node_tests/list_render.js

@@ -1,3 +1,4 @@
+zrequire('scroll_util');
 zrequire('list_render');
 
 // We need these stubs to get by instanceof checks.

frontend_tests/node_tests/scroll_util.js

@@ -93,3 +93,23 @@ run_test('scroll_element_into_container', () => {
     scroll_util.scroll_element_into_container(elem2, container);
     assert.equal(container.scrollTop(), 250 - 100 + 3 + 15);
 });
+
+run_test('get_list_scrolling_container error', () => {
+    const body = {
+        length: 1,
+        is: (sel) => {
+            assert.equal(sel, 'body, html');
+            return true;
+        },
+    };
+
+    blueslip.expect(
+        'error',
+        "Please wrap lists in an element with " +
+        "'max-height' attribute."
+    );
+
+    scroll_util.get_list_scrolling_container(body);
+
+    blueslip.reset();
+});

static/js/list_render.js

@@ -260,24 +260,11 @@ exports.create = function ($container, list, opts) {
     };
 
     widget.set_up_event_handlers = function () {
-        let $nearestScrollingContainer = $container;
-
-        while ($nearestScrollingContainer.length) {
-            if ($nearestScrollingContainer.is("body, html")) {
-                blueslip.warn("Please wrap progressive scrolling lists in an element with 'max-height' attribute. Error found in:\n" + blueslip.preview_node($container));
-                break;
-            }
-
-            if ($nearestScrollingContainer.css("max-height") !== "none") {
-                break;
-            }
-
-            $nearestScrollingContainer = $nearestScrollingContainer.parent();
-        }
+        meta.scroll_container = scroll_util.get_list_scrolling_container($container);
 
         // on scroll of the nearest scrolling container, if it hits the bottom
         // of the container then fetch a new block of items and render them.
-        $nearestScrollingContainer.scroll(function () {
+        meta.scroll_container.scroll(function () {
             if (this.scrollHeight - (this.scrollTop + this.clientHeight) < 10) {
                 widget.render();
             }

static/js/scroll_util.js

@@ -1,3 +1,30 @@
+exports.get_list_scrolling_container = function (container) {
+    /*
+        This is used for list widgets that don't
+        have SimpleBar (in contrast to, say,
+        ui.get_scroll_element().
+    */
+
+    let nearestScrollingContainer = container;
+
+    while (nearestScrollingContainer.length) {
+        if (nearestScrollingContainer.is("body, html")) {
+            blueslip.error(
+                "Please wrap lists in an element with " +
+                "'max-height' attribute.");
+            break;
+        }
+
+        if (nearestScrollingContainer.css("max-height") !== "none") {
+            break;
+        }
+
+        nearestScrollingContainer = nearestScrollingContainer.parent();
+    }
+
+    return nearestScrollingContainer;
+};
+
 exports.scroll_delta = function (opts) {
     const elem_top = opts.elem_top;
     const container_height = opts.container_height;