Fixed disclosure positioning bug

Fixes part of #15852

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Fixed a bug where the time zones listed in Date fields weren’t labelled properly based on the selected date. ([#15805](https://github.com/craftcms/cms/issues/15805))
 - Fixed an error that could occur if a native element property was attempted to be eager-loaded. ([#15822](https://github.com/craftcms/cms/issues/15822))
 - Fixed errors that could occur if a custom source or field condition referenced a custom field whose type had changed. ([#15850](https://github.com/craftcms/cms/issues/15850))
+- Fixed a bug where disclosure menus weren’t sticking to their trigger element as it was scrolled, if it was within a slideout or other inline-scrollable container. ([#15852](https://github.com/craftcms/cms/issues/15852))
 - Fixed a missing authorization vulnerability.
 
 ## 4.12.5 - 2024-09-27

src/web/assets/garnish/src/DisclosureMenu.js

@@ -202,6 +202,10 @@ export default Base.extend(
         'scroll',
         'setContainerPosition'
       );
+      const $scrollParent = this.$trigger.scrollParent();
+      if ($scrollParent.get(0) !== document.body) {
+        this.addListener($scrollParent, 'scroll', 'setContainerPosition');
+      }
       this.addListener(Garnish.$win, 'resize', 'setContainerPosition');
 
       this.$container.velocity('stop');