From 2d67222472f80f251607ae1b720527afceba06ad Mon Sep 17 00:00:00 2001
From: tapan gujjar <tapan@stablehacks.com>
Date: Tue, 5 May 2020 15:40:46 -0400
Subject: [PATCH] Fixed #31522 -- Made admin's SelectBox preserve scroll
 position.

---
 .../admin/static/admin/js/SelectBox.js        |  2 ++
 js_tests/admin/SelectBox.test.js              | 24 +++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/django/contrib/admin/static/admin/js/SelectBox.js b/django/contrib/admin/static/admin/js/SelectBox.js
index e166cf02fc..ace6d9dfb8 100644
--- a/django/contrib/admin/static/admin/js/SelectBox.js
+++ b/django/contrib/admin/static/admin/js/SelectBox.js
@@ -13,6 +13,7 @@
         redisplay: function(id) {
             // Repopulate HTML select box from cache
             const box = document.getElementById(id);
+            const scroll_value_from_top = box.scrollTop;
             box.innerHTML = '';
             for (const node of SelectBox.cache[id]) {
                 if (node.displayed) {
@@ -22,6 +23,7 @@
                     box.appendChild(new_option);
                 }
             }
+            box.scrollTop = scroll_value_from_top;
         },
         filter: function(id, text) {
             // Redisplay the HTML select box, displaying only the choices containing ALL
diff --git a/js_tests/admin/SelectBox.test.js b/js_tests/admin/SelectBox.test.js
index 3de7623691..7d127b5d59 100644
--- a/js_tests/admin/SelectBox.test.js
+++ b/js_tests/admin/SelectBox.test.js
@@ -21,3 +21,27 @@ QUnit.test('filter', function(assert) {
     assert.equal($('#id option').length, 1);
     assert.equal($('#id option').text(), "A");
 });
+
+QUnit.test('preserve scroll position', function(assert) {
+    const $ = django.jQuery;
+    const optionsCount = 100;
+    $('<select id="from_id" multiple></select>').appendTo('#qunit-fixture');
+    $('<select id="to_id" multiple></select>').appendTo('#qunit-fixture');
+    const fromSelectBox = document.getElementById('from_id');
+    const toSelectBox = document.getElementById('to_id');
+    for (let i = 0; i < optionsCount; i++) {
+        fromSelectBox.appendChild(new Option());
+    }
+    SelectBox.init('from_id');
+    SelectBox.init('to_id');
+    const selectedOptions = [97, 98, 99];
+    for (const index of selectedOptions) {
+        fromSelectBox.options[index].selected = true;
+        fromSelectBox.options[index].scrollIntoView();
+    }
+    assert.equal(fromSelectBox.options.length, optionsCount);
+    SelectBox.move('from_id', 'to_id');
+    assert.equal(fromSelectBox.options.length, optionsCount - selectedOptions.length);
+    assert.equal(toSelectBox.options.length, selectedOptions.length);
+    assert.notEqual(fromSelectBox.scrollTop, 0);
+});