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.

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;