diff --git a/packages/frontend/src/components/MkMenu.child.vue b/packages/frontend/src/components/MkMenu.child.vue
index 37c3a3f5e3..ec983c270f 100644
--- a/packages/frontend/src/components/MkMenu.child.vue
+++ b/packages/frontend/src/components/MkMenu.child.vue
@@ -5,7 +5,15 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <template>
 <div ref="el" :class="$style.root">
-	<MkMenu :items="items" :align="align" :width="width" :asDrawer="false" @close="onChildClosed"/>
+	<MkMenu
+		:items="items"
+		:align="align"
+		:width="width"
+		:asDrawer="false"
+		:debugDisablePredictionCone="debugDisablePredictionCone"
+		:debugShowPredictionCone="debugShowPredictionCone"
+		@close="onChildClosed"
+	/>
 </div>
 </template>
 
@@ -19,6 +27,8 @@ const props = defineProps<{
 	anchorElement: HTMLElement;
 	rootElement: HTMLElement;
 	width?: number;
+	debugDisablePredictionCone?: boolean;
+	debugShowPredictionCone?: boolean;
 }>();
 
 const emit = defineEmits<{
@@ -80,6 +90,7 @@ onUnmounted(() => {
 });
 
 defineExpose({
+	rootElement: el,
 	checkHit: (ev: MouseEvent) => {
 		return (ev.target === el.value || el.value?.contains(ev.target as Node));
 	},
diff --git a/packages/frontend/src/components/MkMenu.vue b/packages/frontend/src/components/MkMenu.vue
index b618dab6b2..26db8e1b5b 100644
--- a/packages/frontend/src/components/MkMenu.vue
+++ b/packages/frontend/src/components/MkMenu.vue
@@ -27,6 +27,8 @@ SPDX-License-Identifier: AGPL-3.0-only
 		}"
 		@keydown.stop="() => {}"
 		@contextmenu.self.prevent="() => {}"
+		@mousemove.passive="onMouseMove"
+		@mouseleave.passive="onMouseLeave"
 	>
 		<template v-for="item in (items2 ?? [])">
 			<div v-if="item.type === 'divider'" role="separator" tabindex="-1" :class="$style.divider"></div>
@@ -169,6 +171,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 				tabindex="0"
 				:class="['_button', $style.item, $style.parent, { [$style.active]: childShowingItem === item }]"
 				@mouseenter.prevent="preferClick ? null : showChildren(item, $event)"
+				@mousemove="parentMouseMove"
 				@keydown.enter.prevent="preferClick ? null : showChildren(item, $event)"
 				@click.prevent="!preferClick ? null : showChildren(item, $event)"
 			>
@@ -206,15 +209,30 @@ SPDX-License-Identifier: AGPL-3.0-only
 		<span v-if="items2 == null || items2.length === 0" tabindex="-1" :class="[$style.none, $style.item]">
 			<span>{{ i18n.ts.none }}</span>
 		</span>
+
+		<div
+			:class="[$style.guard, { [$style.showGuard]: debugShowPredictionCone }]"
+			:style="{ clipPath: guardPolygon, top: guard.top + 'px' }"
+			@mousemove="guardMouseMove"
+		></div>
 	</div>
-	<div v-if="childMenu">
-		<XChild ref="child" :items="childMenu" :anchorElement="childTarget!" :rootElement="itemsEl!" @actioned="childActioned" @closed="closeChild"/>
-	</div>
+
+	<XChild
+		v-if="childMenu" :key="childMenuKey"
+		ref="child"
+		:items="childMenu"
+		:anchorElement="childTarget!"
+		:rootElement="itemsEl!"
+		:debugDisablePredictionCone="props.debugDisablePredictionCone"
+		:debugShowPredictionCone="props.debugShowPredictionCone"
+		@actioned="childActioned"
+		@closed="closeChild"
+	/>
 </div>
 </template>
 
 <script lang="ts">
-import { computed, defineAsyncComponent, inject, nextTick, onBeforeUnmount, onMounted, ref, useTemplateRef, unref, watch, shallowRef } from 'vue';
+import { computed, defineAsyncComponent, inject, nextTick, onBeforeUnmount, onMounted, ref, useTemplateRef, unref, watch, shallowRef, reactive } from 'vue';
 import type { MenuItem, InnerMenuItem, MenuPending, MenuAction, MenuSwitch, MenuRadio, MenuRadioOption, MenuParent } from '@/types/menu.js';
 import type { Keymap } from '@/utility/hotkey.js';
 import MkSwitchButton from '@/components/MkSwitch.button.vue';
@@ -236,6 +254,8 @@ const props = defineProps<{
 	align?: 'center' | string;
 	width?: number;
 	maxHeight?: number;
+	debugDisablePredictionCone?: boolean;
+	debugShowPredictionCone?: boolean;
 }>();
 
 const emit = defineEmits<{
@@ -292,6 +312,7 @@ watch(() => props.items, () => {
 });
 
 const childMenu = ref<MenuItem[] | null>();
+const childMenuKey = ref(0);
 const childTarget = shallowRef<HTMLElement>();
 
 function closeChild() {
@@ -348,6 +369,7 @@ async function showRadioOptions(item: MenuRadio, ev: MouseEvent | PointerEvent |
 	} else {
 		childTarget.value = (ev.currentTarget ?? ev.target) as HTMLElement;
 		childMenu.value = children;
+		childMenuKey.value++;
 		childShowingItem.value = item;
 	}
 }
@@ -378,6 +400,7 @@ async function showChildren(item: MenuParent, ev: MouseEvent | PointerEvent | Ke
 		childTarget.value = (ev.currentTarget ?? ev.target) as HTMLElement;
 		// これでもリアクティビティは保たれる
 		childMenu.value = children;
+		childMenuKey.value++;
 		childShowingItem.value = item;
 	}
 }
@@ -472,6 +495,67 @@ onMounted(() => {
 onBeforeUnmount(() => {
 	disposeHandlers();
 });
+
+const guard = reactive({
+	enabled: false,
+	top: 0,
+	cursorSideX: 0,
+	cursorSideY: 0,
+	childSideTopY: 0,
+	childSideBottomY: 0,
+	direction: 'toRight',
+});
+
+const guardPolygon = computed(() =>
+	guard.enabled
+		? guard.direction === 'toRight'
+			? `polygon(${guard.cursorSideX}px ${guard.cursorSideY}px, 101% ${guard.childSideTopY}px, 101% ${guard.childSideBottomY}px)` // ぴったり端に100%で覆ってもなぜか端でカーソルのイベントが後ろに貫通するので1%だけ伸ばす
+			: `polygon(0% ${guard.childSideTopY}px, 0% ${guard.childSideBottomY}px, ${guard.cursorSideX}px ${guard.cursorSideY}px)`
+		: 'polygon(0 0, 0 0, 0 0)',
+);
+
+function parentMouseMove(ev: MouseEvent) {
+	if (props.debugDisablePredictionCone) return;
+	if (isTouchUsing) return;
+	if (child.value == null || child.value.rootElement == null) return;
+
+	ev.stopPropagation();
+
+	const itemBounding = (ev.currentTarget as HTMLElement).getBoundingClientRect();
+	const rootBounding = itemsEl.value!.getBoundingClientRect();
+	const childBounding = child.value.rootElement.getBoundingClientRect();
+	const isChildRight = childBounding.left > rootBounding.left;
+
+	const CURSOR_SIDE_X_PADDING = 3; // (px)
+	const CHILD_SIDE_Y_PADDING_BASE = 70; // (px)
+	const CHILD_SIDE_Y_PADDING_EXTEND = 30; // (px)
+	const SCALE_FACTOR_COMPUTE_DISTANCE = 300; // コーンの広さが最大になる距離(px)
+	const localMouseX = ev.clientX - itemBounding.left;
+	const localMouseY = ev.clientY - rootBounding.top;
+	const scaleFactor = isChildRight ? Math.min((itemBounding.width - localMouseX), SCALE_FACTOR_COMPUTE_DISTANCE) / SCALE_FACTOR_COMPUTE_DISTANCE : Math.min(localMouseX, SCALE_FACTOR_COMPUTE_DISTANCE) / SCALE_FACTOR_COMPUTE_DISTANCE;
+	const cursorSideXPadding = isChildRight ? CURSOR_SIDE_X_PADDING : -CURSOR_SIDE_X_PADDING;
+	const childSideYPadding = CHILD_SIDE_Y_PADDING_BASE + (CHILD_SIDE_Y_PADDING_EXTEND * scaleFactor);
+
+	guard.enabled = true;
+	guard.top = itemsEl.value!.scrollTop;
+	guard.cursorSideX = localMouseX - cursorSideXPadding;
+	guard.cursorSideY = localMouseY;
+	guard.childSideTopY = (childBounding.top - rootBounding.top) - childSideYPadding;
+	guard.childSideBottomY = (childBounding.bottom - rootBounding.top) + childSideYPadding;
+	guard.direction = isChildRight ? 'toRight' : 'toLeft';
+}
+
+function onMouseLeave() {
+	guard.enabled = false;
+}
+
+function onMouseMove() {
+	guard.enabled = false;
+}
+
+function guardMouseMove(ev: MouseEvent) {
+	ev.stopPropagation();
+}
 </script>
 
 <style lang="scss" module>
@@ -592,6 +676,8 @@ onBeforeUnmount(() => {
 		&:focus-visible:active,
 		&:focus-visible.active {
 			color: var(--menuHoverFg, var(--MI_THEME-accent));
+			position: relative;
+			z-index: 10; // guardより上にする
 
 			&::before {
 				background-color: var(--menuHoverBg, var(--MI_THEME-accentedBg));
@@ -744,4 +830,20 @@ onBeforeUnmount(() => {
 		}
 	}
 }
+
+.guard {
+	position: absolute;
+	left: 0;
+	width: 100%;
+	height: 100%;
+	cursor: pointer;
+
+	&.showGuard {
+		background: #0f04;
+
+		&:hover {
+			background: #f004;
+		}
+	}
+}
 </style>
diff --git a/packages/frontend/src/components/MkPopupMenu.vue b/packages/frontend/src/components/MkPopupMenu.vue
index 4942ffe232..11b116d430 100644
--- a/packages/frontend/src/components/MkPopupMenu.vue
+++ b/packages/frontend/src/components/MkPopupMenu.vue
@@ -4,8 +4,31 @@ SPDX-License-Identifier: AGPL-3.0-only
 -->
 
 <template>
-<MkModal ref="modal" v-slot="{ type, maxHeight }" :manualShowing="manualShowing" :zPriority="'high'" :anchorElement="anchorElement" :transparentBg="true" :returnFocusTo="returnFocusTo" @click="click" @close="onModalClose" @closed="onModalClosed">
-	<MkMenu :items="items" :align="align" :width="width" :max-height="maxHeight" :asDrawer="type === 'drawer'" :returnFocusTo="returnFocusTo" :class="{ [$style.drawer]: type === 'drawer' }" @close="onMenuClose" @hide="hide"/>
+<MkModal
+	ref="modal"
+	v-slot="{ type, maxHeight }"
+	:manualShowing="manualShowing"
+	:zPriority="'high'"
+	:anchorElement="anchorElement"
+	:transparentBg="true"
+	:returnFocusTo="returnFocusTo"
+	@click="click"
+	@close="onModalClose"
+	@closed="onModalClosed"
+>
+	<MkMenu
+		:items="items"
+		:align="align"
+		:width="width"
+		:max-height="maxHeight"
+		:asDrawer="type === 'drawer'"
+		:returnFocusTo="returnFocusTo"
+		:debugDisablePredictionCone="debugDisablePredictionCone"
+		:debugShowPredictionCone="debugShowPredictionCone"
+		:class="{ [$style.drawer]: type === 'drawer' }"
+		@close="onMenuClose"
+		@hide="hide"
+	/>
 </MkModal>
 </template>
 
@@ -21,6 +44,8 @@ defineProps<{
 	width?: number;
 	anchorElement?: HTMLElement | null;
 	returnFocusTo?: HTMLElement | null;
+	debugDisablePredictionCone?: boolean;
+	debugShowPredictionCone?: boolean;
 }>();
 
 const emit = defineEmits<{
diff --git a/packages/frontend/src/os.ts b/packages/frontend/src/os.ts
index f7b59612c4..8e78127e8e 100644
--- a/packages/frontend/src/os.ts
+++ b/packages/frontend/src/os.ts
@@ -640,6 +640,8 @@ export function popupMenu(items: (MenuItem | null)[], anchorElement?: HTMLElemen
 	width?: number;
 	onClosing?: () => void;
 	onClosed?: () => void;
+	debugDisablePredictionCone?: boolean;
+	debugShowPredictionCone?: boolean;
 }): Promise<void> {
 	if (!(anchorElement instanceof HTMLElement)) {
 		anchorElement = null;
@@ -653,6 +655,8 @@ export function popupMenu(items: (MenuItem | null)[], anchorElement?: HTMLElemen
 			width: options?.width,
 			align: options?.align,
 			returnFocusTo,
+			debugDisablePredictionCone: options?.debugDisablePredictionCone,
+			debugShowPredictionCone: options?.debugShowPredictionCone,
 		}, {
 			closed: () => {
 				resolve();
diff --git a/packages/frontend/src/pages/debug.vue b/packages/frontend/src/pages/debug.vue
index 9c0761f0b1..44418d613b 100644
--- a/packages/frontend/src/pages/debug.vue
+++ b/packages/frontend/src/pages/debug.vue
@@ -7,30 +7,46 @@ SPDX-License-Identifier: AGPL-3.0-only
 <PageWithHeader>
 	<div class="_spacer" style="--MI_SPACER-w: 600px;">
 		<div class="_gaps_m">
-			<MkResult v-if="resultType === 'empty'" type="empty"/>
-			<MkResult v-if="resultType === 'notFound'" type="notFound"/>
-			<MkResult v-if="resultType === 'error'" type="error"/>
-			<MkSelect
-				v-model="resultType" :items="resultTypeDef"
-			></MkSelect>
+			<MkFolder>
+				<template #label>Icons</template>
 
-			<MkSystemIcon v-if="iconType === 'info'" type="info" style="width: 150px;"/>
-			<MkSystemIcon v-if="iconType === 'question'" type="question" style="width: 150px;"/>
-			<MkSystemIcon v-if="iconType === 'success'" type="success" style="width: 150px;"/>
-			<MkSystemIcon v-if="iconType === 'warn'" type="warn" style="width: 150px;"/>
-			<MkSystemIcon v-if="iconType === 'error'" type="error" style="width: 150px;"/>
-			<MkSystemIcon v-if="iconType === 'waiting'" type="waiting" style="width: 150px;"/>
-			<MkSelect
-				v-model="iconType" :items="iconTypeDef"
-			></MkSelect>
+				<div class="_gaps_m">
+					<MkResult v-if="resultType === 'empty'" type="empty"/>
+					<MkResult v-if="resultType === 'notFound'" type="notFound"/>
+					<MkResult v-if="resultType === 'error'" type="error"/>
+					<MkSelect
+						v-model="resultType" :items="resultTypeDef"
+					></MkSelect>
 
-			<div class="_buttons">
-				<MkButton @click="os.alert({ type: 'error', title: 'Error', text: 'error' })">Error</MkButton>
-				<MkButton @click="os.alert({ type: 'warning', title: 'Warning', text: 'warning' })">Warning</MkButton>
-				<MkButton @click="os.alert({ type: 'info', title: 'Info', text: 'info' })">Info</MkButton>
-				<MkButton @click="os.alert({ type: 'success', title: 'Success', text: 'success' })">Success</MkButton>
-				<MkButton @click="os.alert({ type: 'question', title: 'Question', text: 'question' })">Question</MkButton>
-			</div>
+					<MkSystemIcon v-if="iconType === 'info'" type="info" style="width: 150px;"/>
+					<MkSystemIcon v-if="iconType === 'question'" type="question" style="width: 150px;"/>
+					<MkSystemIcon v-if="iconType === 'success'" type="success" style="width: 150px;"/>
+					<MkSystemIcon v-if="iconType === 'warn'" type="warn" style="width: 150px;"/>
+					<MkSystemIcon v-if="iconType === 'error'" type="error" style="width: 150px;"/>
+					<MkSystemIcon v-if="iconType === 'waiting'" type="waiting" style="width: 150px;"/>
+					<MkSelect
+						v-model="iconType" :items="iconTypeDef"
+					></MkSelect>
+
+					<div class="_buttons">
+						<MkButton @click="os.alert({ type: 'error', title: 'Error', text: 'error' })">Error</MkButton>
+						<MkButton @click="os.alert({ type: 'warning', title: 'Warning', text: 'warning' })">Warning</MkButton>
+						<MkButton @click="os.alert({ type: 'info', title: 'Info', text: 'info' })">Info</MkButton>
+						<MkButton @click="os.alert({ type: 'success', title: 'Success', text: 'success' })">Success</MkButton>
+						<MkButton @click="os.alert({ type: 'question', title: 'Question', text: 'question' })">Question</MkButton>
+					</div>
+				</div>
+			</MkFolder>
+
+			<MkFolder>
+				<template #label>Nested menu guard (a.k.a "prediction cone")</template>
+
+				<div class="_buttons">
+					<MkButton @click="select($event, false, false)">select without guard</MkButton>
+					<MkButton @click="select($event, true, false)">select with guard</MkButton>
+					<MkButton @click="select($event, true, true)">select with guard (visualize)</MkButton>
+				</div>
+			</MkFolder>
 		</div>
 	</div>
 </PageWithHeader>
@@ -47,6 +63,7 @@ import MkSelect from '@/components/MkSelect.vue';
 import MkButton from '@/components/MkButton.vue';
 import { useMkSelect } from '@/composables/use-mkselect.js';
 import * as os from '@/os.js';
+import MkFolder from '@/components/MkFolder.vue';
 
 const {
 	model: resultType,
@@ -74,6 +91,64 @@ const {
 	initialValue: 'info',
 });
 
+function select(ev: PointerEvent, enablePredictionCone: boolean, showPredictionCone: boolean) {
+	os.popupMenu([
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+		] },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+		] },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+		] },
+		{ text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', action: () => {} },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+		] },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+		] },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+			{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+				{ text: 'Option', action: () => {} },
+				{ text: 'Option', action: () => {} },
+				{ text: 'Option', action: () => {} },
+				{ text: 'Option', action: () => {} },
+				{ text: 'Option', action: () => {} },
+			] },
+			{ text: 'Option', action: () => {} },
+			{ text: 'Option', action: () => {} },
+		] },
+		{ type: 'parent', text: 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX', children: [
+			{ text: 'Option', action: () => {} },
+		] },
+	], ev.currentTarget ?? ev.target, {
+		debugDisablePredictionCone: !enablePredictionCone,
+		debugShowPredictionCone: showPredictionCone,
+	}).then((value) => {
+		console.log('Selected:', value);
+	});
+}
+
 definePage(() => ({
 	title: 'DEBUG ROOM',
 	icon: 'ti ti-help-circle',