fix(plugin-chart-handlebars): preserve template on explore open

superset-frontend/plugins/plugin-chart-handlebars/src/plugin/controls/handlebarTemplate.tsx

@@ -106,8 +106,8 @@ export const handlebarsTemplateControlSetItem: ControlSetItem = {
     valueKey: null,
 
     validators: [validateNonEmpty],
-    mapStateToProps: ({ controls }) => ({
-      value: controls?.handlebars_template?.value,
+    mapStateToProps: ({ form_data }) => ({
+      value: form_data?.handlebarsTemplate ?? form_data?.handlebars_template,
     }),
   },
 };

superset-frontend/plugins/plugin-chart-handlebars/src/plugin/controls/style.tsx

@@ -87,8 +87,8 @@ export const styleControlSetItem: ControlSetItem = {
     valueKey: null,
 
     validators: [],
-    mapStateToProps: ({ controls, common }) => ({
-      value: controls?.handlebars_template?.value,
+    mapStateToProps: ({ form_data, common }) => ({
+      value: form_data?.styleTemplate ?? form_data?.style_template,
       htmlSanitization: common?.conf?.HTML_SANITIZATION ?? true,
     }),
   },

superset-frontend/plugins/plugin-chart-handlebars/test/plugin/controls.test.ts

@@ -0,0 +1,116 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+import {
+  ControlPanelState,
+  ControlState,
+  CustomControlItem,
+} from '@superset-ui/chart-controls';
+import { QueryFormData } from '@superset-ui/core';
+import { handlebarsTemplateControlSetItem } from '../../src/plugin/controls/handlebarTemplate';
+import { styleControlSetItem } from '../../src/plugin/controls/style';
+
+const handlebarsConfig = (handlebarsTemplateControlSetItem as CustomControlItem)
+  .config;
+const styleConfig = (styleControlSetItem as CustomControlItem).config;
+
+const buildState = (form_data: Partial<QueryFormData>) =>
+  ({
+    form_data: form_data as QueryFormData,
+    controls: {},
+    datasource: null,
+    common: { conf: { HTML_SANITIZATION: true } },
+    slice: { slice_id: 1 },
+  }) as unknown as ControlPanelState;
+
+const CUSTOM = '<div>custom template</div>';
+const CUSTOM_CSS = '.foo { color: red; }';
+
+test('handlebarsTemplate mapStateToProps reads snake_case handlebars_template (MCP-created charts)', () => {
+  const result = handlebarsConfig.mapStateToProps!(
+    buildState({ handlebars_template: CUSTOM } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe(CUSTOM);
+});
+
+test('handlebarsTemplate mapStateToProps reads camelCase handlebarsTemplate (UI-created charts)', () => {
+  const result = handlebarsConfig.mapStateToProps!(
+    buildState({ handlebarsTemplate: CUSTOM } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe(CUSTOM);
+});
+
+test('handlebarsTemplate mapStateToProps prefers camelCase when both keys present (latest edit wins over legacy snake_case)', () => {
+  const result = handlebarsConfig.mapStateToProps!(
+    buildState({
+      handlebars_template: 'stale legacy value',
+      handlebarsTemplate: 'latest edit',
+    } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe('latest edit');
+});
+
+test('handlebarsTemplate mapStateToProps returns undefined when no template stored (allows default)', () => {
+  const result = handlebarsConfig.mapStateToProps!(
+    buildState({}),
+    {} as ControlState,
+  );
+  expect(result.value).toBeUndefined();
+});
+
+test('styleTemplate mapStateToProps reads camelCase styleTemplate (MCP and UI charts)', () => {
+  const result = styleConfig.mapStateToProps!(
+    buildState({ styleTemplate: CUSTOM_CSS } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe(CUSTOM_CSS);
+  expect(result.htmlSanitization).toBe(true);
+});
+
+test('styleTemplate mapStateToProps prefers camelCase when both keys present', () => {
+  const result = styleConfig.mapStateToProps!(
+    buildState({
+      style_template: 'stale',
+      styleTemplate: 'latest',
+    } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe('latest');
+});
+
+test('styleTemplate mapStateToProps reads snake_case style_template as fallback', () => {
+  const result = styleConfig.mapStateToProps!(
+    buildState({ style_template: CUSTOM_CSS } as Partial<QueryFormData>),
+    {} as ControlState,
+  );
+  expect(result.value).toBe(CUSTOM_CSS);
+});
+
+test('styleTemplate mapStateToProps uses HTML_SANITIZATION=false from config', () => {
+  const result = styleConfig.mapStateToProps!(
+    {
+      ...buildState({}),
+      common: { conf: { HTML_SANITIZATION: false } },
+    } as unknown as ControlPanelState,
+    {} as ControlState,
+  );
+  expect(result.htmlSanitization).toBe(false);
+});