diff --git a/packages/ui-default/components/problemconfig/ProblemConfigEditor.tsx b/packages/ui-default/components/problemconfig/ProblemConfigEditor.tsx
index 15df4898f..934e7b074 100644
--- a/packages/ui-default/components/problemconfig/ProblemConfigEditor.tsx
+++ b/packages/ui-default/components/problemconfig/ProblemConfigEditor.tsx
@@ -96,7 +96,7 @@ export default connect(mapStateToProps, mapDispatchToProps)(class MonacoEditor e
   async componentDidMount() {
     const { monaco } = await load(['yaml']);
     const uri = monaco.Uri.parse('hydro://problem/file/config.yaml');
-    this.model = monaco.editor.createModel(yaml.dump(configYamlFormat(this.props.config)), 'yaml', uri);
+    this.model = monaco.editor.createModel(yaml.dump(configYamlFormat(this.props.config), { noArrayIndent: true }), 'yaml', uri);
     this.vjEditor = Editor.getOrConstruct($(this.containerElement), {
       language: 'yaml',
       model: this.model,
@@ -112,7 +112,7 @@ export default connect(mapStateToProps, mapDispatchToProps)(class MonacoEditor e
 
   componentDidUpdate(prevProps) {
     if (this.__preventUpdate || !this.model || !this.props.config.__valid) return;
-    if (yaml.dump(prevProps.config) === yaml.dump(this.props.config)) return;
+    if (yaml.dump(prevProps.config, { noArrayIndent: true }) === yaml.dump(this.props.config, { noArrayIndent: true })) return;
     const curValue = this.model.getValue();
     const pending = configYamlFormat(this.props.config);
     try {
@@ -120,7 +120,7 @@ export default connect(mapStateToProps, mapDispatchToProps)(class MonacoEditor e
       if (isEqual(curConfig, pending)) return;
     } catch { }
     this.__preventFormat = true;
-    const diff = diffLines(curValue, yaml.dump(pending));
+    const diff = diffLines(curValue, yaml.dump(pending, { noArrayIndent: true }));
     const ops = [];
     let cursor = 1;
     for (const line of diff) {
diff --git a/packages/ui-default/components/problemconfig/ProblemConfigTree.tsx b/packages/ui-default/components/problemconfig/ProblemConfigTree.tsx
index 873e29b3d..f83547190 100644
--- a/packages/ui-default/components/problemconfig/ProblemConfigTree.tsx
+++ b/packages/ui-default/components/problemconfig/ProblemConfigTree.tsx
@@ -1,4 +1,5 @@
-import { Icon, TreeNode } from '@blueprintjs/core';
+import { Button, Icon, TreeNode } from '@blueprintjs/core';
+import { Popover2 } from '@blueprintjs/popover2';
 import { normalizeSubtasks, readSubtasksFromFiles } from '@hydrooj/utils/lib/common';
 import { TestCaseConfig } from 'hydrooj';
 import { isEqual } from 'lodash';
@@ -19,9 +20,9 @@ interface TestcasesDndItem {
 export function SubtaskNode(props: { subtaskId: number }) {
   const { subtaskId } = props;
   const subtaskIds = useSelector((s: RootState) => Object.values(s.config?.subtasks || []).map((i) => i.id), isEqual);
-  const clen = useSelector((state: RootState) => (subtaskId === -1
-    ? state.config.__cases.length
-    : state.config.subtasks.find((i) => i.id === subtaskId).cases?.length || 0));
+  const cases = useSelector((state: RootState) => (subtaskId === -1
+    ? state.config.__cases
+    : state.config.subtasks.find((i) => i.id === subtaskId).cases || []));
   const time = useSelector((s: RootState) => s.config?.time);
   const memory = useSelector((s: RootState) => s.config?.memory);
   const dispatch = useDispatch();
@@ -43,13 +44,37 @@ export function SubtaskNode(props: { subtaskId: number }) {
     },
   }));
 
+  function deleteSubtask() {
+    dispatch({
+      type: 'problemconfig/moveTestcases',
+      payload: {
+        target: -1,
+        source: subtaskId,
+        cases,
+      },
+    });
+    dispatch({
+      type: 'problemconfig/deleteSubtask',
+      id: subtaskId,
+    });
+  }
+
   return (
     <li className="bp4-tree-node bp4-tree-node-expanded">
       {subtaskId !== -1 && <div className="bp4-tree-node-content" onClick={() => setExpand((e) => !e)}>
         <Icon icon={expand ? 'folder-open' : 'folder-close'} />&nbsp;
         <span className="bp4-tree-node-label">Subtask {subtaskId}</span>
-        <span className="bp4-tree-node-secondary-label">
-          <Icon icon="trash"></Icon>
+        <span className="bp4-tree-node-secondary-label" onClick={(ev) => ev.stopPropagation()}>
+          <Popover2
+            content={<div style={{ padding: 20 }}>
+              <b>Are you sure you want to delete this subtask?</b>
+              <div style={{ display: 'flex', justifyContent: 'flex-end', marginTop: 15 }}>
+                <Button intent="danger" onClick={deleteSubtask}>Delete</Button>
+              </div>
+            </div>}
+          >
+            <Icon icon="trash" />
+          </Popover2>
         </span>
       </div>}
       <ul className="bp4-tree-node-list" ref={drop}>
@@ -61,10 +86,10 @@ export function SubtaskNode(props: { subtaskId: number }) {
             id={`s${subtaskId}`}
             onClick={() => setExpand(false)}
             icon="layers"
-            label={<>&nbsp;{clen} testcases.</>}
+            label={<>&nbsp;{cases.length} testcases.</>}
             path={[0]}
           />}
-        {!clen && (
+        {!cases.length && (
           <li className="bp4-tree-node">
             <div className="bp4-tree-node-content">
               <span className="bp4-tree-node-caret-none bp4-icon-standard"></span>
diff --git a/packages/ui-default/components/problemconfig/reducer/config.ts b/packages/ui-default/components/problemconfig/reducer/config.ts
index f90d1d7c5..bff419ca7 100644
--- a/packages/ui-default/components/problemconfig/reducer/config.ts
+++ b/packages/ui-default/components/problemconfig/reducer/config.ts
@@ -125,6 +125,10 @@ export default function reducer(state = {
       }
       return { ...state, subtasks, __cases };
     }
+    case 'problemconfig/deleteSubtask': {
+      const subtasks = state.subtasks.filter((i) => i.id !== action.id);
+      return { ...state, subtasks };
+    }
     default:
       return state;
   }