[6.x] Preserve settings input and errors after failed saves

CHANGELOG.md

@@ -34,6 +34,7 @@
 - Migrated the reassign entries, replace relations, and replace references modals to the Form API. ([#19589](https://github.com/craftcms/cms/pull/19589))
 - Restored Score sorting for element index searches and preserved previous sort fields as tie-breakers. ([#19731](https://github.com/craftcms/cms/pull/19731))
 - Fixed a bug where the Edit and View bulk actions did nothing on element indexes. ([#19747](https://github.com/craftcms/cms/pull/19747))
+- Fixed a bug where failed settings saves could lose input and validation errors when a successful save would redirect to another page. ([#19775](https://github.com/craftcms/cms/pull/19775))
 - Fixed a bug where money inputs could be hidden within fields or submit their values under the wrong name. ([#19767](https://github.com/craftcms/cms/pull/19767))
 - Fixed a bug where visually-hidden labels, headings, and status messages were visible on some control panel pages.
 - Fixed a bug where `<craft-input>` fields with a `maxlength` weren’t sized to fit it, which made the element index pagination’s page input overlap the “Next page” button.

resources/js/common/types/index.ts

@@ -231,7 +231,7 @@ export type ActionItems = Array<ActionItem>;
 export interface FormSaveOptions {
   redirect?: boolean;
   data?: FormValues;
-  preserveState?: boolean;
+  preserveState?: boolean | 'errors';
 }
 
 export interface EntryType {

resources/js/modules/settings/composables/useSettingsSave.ts

@@ -113,7 +113,8 @@ export function useSettingsSave<T extends object>(
     data: extraData = {},
     // Reset page state when this screen sends the user elsewhere, while saves
     // that remain on the current screen keep their local state by default.
-    preserveState = !(redirect && redirectUrl.value),
+    // Validation failures preserve input and errors instead of remounting.
+    preserveState = redirect && redirectUrl.value ? 'errors' : true,
   }: FormSaveOptions = {}) {
     options.onBeforeSave?.();