diff --git a/packages/frontend/src/components/MkContainer.vue b/packages/frontend/src/components/MkContainer.vue
index 8ab01d7db8..f513795c56 100644
--- a/packages/frontend/src/components/MkContainer.vue
+++ b/packages/frontend/src/components/MkContainer.vue
@@ -64,26 +64,30 @@ const showBody = ref(props.expanded);
 const ignoreOmit = ref(false);
 const omitted = ref(false);
 
-function enter(el) {
+function enter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
-	el.style.height = 0;
+	el.style.height = '0';
 	el.offsetHeight; // reflow
-	el.style.height = Math.min(elementHeight, props.maxHeight ?? Infinity) + 'px';
+	el.style.height = `${Math.min(elementHeight, props.maxHeight ?? Infinity)}px`;
 }
 
-function afterEnter(el) {
-	el.style.height = null;
+function afterEnter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
-function leave(el) {
+function leave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
-	el.style.height = elementHeight + 'px';
+	el.style.height = `${elementHeight}px`;
 	el.offsetHeight; // reflow
-	el.style.height = 0;
+	el.style.height = '0';
 }
 
-function afterLeave(el) {
-	el.style.height = null;
+function afterLeave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
 const calcOmit = () => {
diff --git a/packages/frontend/src/components/MkDateSeparatedList.vue b/packages/frontend/src/components/MkDateSeparatedList.vue
index f04e5cf7c6..9c75f91cb2 100644
--- a/packages/frontend/src/components/MkDateSeparatedList.vue
+++ b/packages/frontend/src/components/MkDateSeparatedList.vue
@@ -128,14 +128,14 @@ export default defineComponent({
 			return children;
 		};
 
-		function onBeforeLeave(element: Element) {
-			const el = element as HTMLElement;
+		function onBeforeLeave(el: Element) {
+			if (!(el instanceof HTMLElement)) return;
 			el.style.top = `${el.offsetTop}px`;
 			el.style.left = `${el.offsetLeft}px`;
 		}
 
-		function onLeaveCancelled(element: Element) {
-			const el = element as HTMLElement;
+		function onLeaveCancelled(el: Element) {
+			if (!(el instanceof HTMLElement)) return;
 			el.style.top = '';
 			el.style.left = '';
 		}
diff --git a/packages/frontend/src/components/MkFoldableSection.vue b/packages/frontend/src/components/MkFoldableSection.vue
index 1717f8fc98..fb1b5220fb 100644
--- a/packages/frontend/src/components/MkFoldableSection.vue
+++ b/packages/frontend/src/components/MkFoldableSection.vue
@@ -5,7 +5,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <template>
 <div ref="rootEl" :class="$style.root">
-	<header :class="$style.header" class="_button" :style="{ background: bg }" @click="showBody = !showBody">
+	<header :class="$style.header" class="_button" @click="showBody = !showBody">
 		<div :class="$style.title"><div><slot name="header"></slot></div></div>
 		<div :class="$style.divider"></div>
 		<button class="_button" :class="$style.button">
@@ -32,21 +32,23 @@ SPDX-License-Identifier: AGPL-3.0-only
 
 <script lang="ts" setup>
 import { onMounted, ref, shallowRef, watch } from 'vue';
-import tinycolor from 'tinycolor2';
 import { miLocalStorage } from '@/local-storage.js';
 import { defaultStore } from '@/store.js';
+import { getBgColor } from '@/scripts/get-bg-color.js';
 
 const miLocalStoragePrefix = 'ui:folder:' as const;
 
 const props = withDefaults(defineProps<{
 	expanded?: boolean;
-	persistKey?: string;
+	persistKey?: string | null;
 }>(), {
 	expanded: true,
+	persistKey: null,
 });
 
-const rootEl = shallowRef<HTMLDivElement>();
-const bg = ref<string>();
+const rootEl = shallowRef<HTMLElement>();
+const parentBg = ref<string | null>(null);
+// eslint-disable-next-line vue/no-setup-props-reactivity-loss
 const showBody = ref((props.persistKey && miLocalStorage.getItem(`${miLocalStoragePrefix}${props.persistKey}`)) ? (miLocalStorage.getItem(`${miLocalStoragePrefix}${props.persistKey}`) === 't') : props.expanded);
 
 watch(showBody, () => {
@@ -55,47 +57,34 @@ watch(showBody, () => {
 	}
 });
 
-function enter(element: Element) {
-	const el = element as HTMLElement;
+function enter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
 	el.style.height = '0';
 	el.offsetHeight; // reflow
-	el.style.height = elementHeight + 'px';
+	el.style.height = `${elementHeight}px`;
 }
 
-function afterEnter(element: Element) {
-	const el = element as HTMLElement;
-	el.style.height = 'unset';
+function afterEnter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
-function leave(element: Element) {
-	const el = element as HTMLElement;
+function leave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
-	el.style.height = elementHeight + 'px';
+	el.style.height = `${elementHeight}px`;
 	el.offsetHeight; // reflow
 	el.style.height = '0';
 }
 
-function afterLeave(element: Element) {
-	const el = element as HTMLElement;
-	el.style.height = 'unset';
+function afterLeave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
 onMounted(() => {
-	function getParentBg(el?: HTMLElement | null): string {
-		if (el == null || el.tagName === 'BODY') return 'var(--MI_THEME-bg)';
-		const background = el.style.background || el.style.backgroundColor;
-		if (background) {
-			return background;
-		} else {
-			return getParentBg(el.parentElement);
-		}
-	}
-
-	const rawBg = getParentBg(rootEl.value);
-	const _bg = tinycolor(rawBg.startsWith('var(') ? getComputedStyle(document.documentElement).getPropertyValue(rawBg.slice(4, -1)) : rawBg);
-	_bg.setAlpha(0.85);
-	bg.value = _bg.toRgbString();
+	parentBg.value = getBgColor(rootEl.value?.parentElement);
 });
 </script>
 
@@ -121,6 +110,7 @@ onMounted(() => {
 	top: var(--MI-stickyTop, 0px);
 	-webkit-backdrop-filter: var(--MI-blur, blur(8px));
 	backdrop-filter: var(--MI-blur, blur(20px));
+	background-color: color(from v-bind("parentBg ?? 'var(--bg)'") srgb r g b / 0.85);
 }
 
 .title {
diff --git a/packages/frontend/src/components/MkFolder.vue b/packages/frontend/src/components/MkFolder.vue
index 5f9500d923..7bdc06a8b4 100644
--- a/packages/frontend/src/components/MkFolder.vue
+++ b/packages/frontend/src/components/MkFolder.vue
@@ -56,8 +56,9 @@ SPDX-License-Identifier: AGPL-3.0-only
 </template>
 
 <script lang="ts" setup>
-import { nextTick, onMounted, shallowRef, ref } from 'vue';
+import { nextTick, onMounted, ref, shallowRef } from 'vue';
 import { defaultStore } from '@/store.js';
+import { getBgColor } from '@/scripts/get-bg-color.js';
 
 const props = withDefaults(defineProps<{
 	defaultOpen?: boolean;
@@ -69,40 +70,35 @@ const props = withDefaults(defineProps<{
 	withSpacer: true,
 });
 
-const getBgColor = (el: HTMLElement) => {
-	const style = window.getComputedStyle(el);
-	if (style.backgroundColor && !['rgba(0, 0, 0, 0)', 'rgba(0,0,0,0)', 'transparent'].includes(style.backgroundColor)) {
-		return style.backgroundColor;
-	} else {
-		return el.parentElement ? getBgColor(el.parentElement) : 'transparent';
-	}
-};
-
 const rootEl = shallowRef<HTMLElement>();
 const bgSame = ref(false);
 const opened = ref(props.defaultOpen);
 const openedAtLeastOnce = ref(props.defaultOpen);
 
-function enter(el) {
+function enter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
-	el.style.height = 0;
+	el.style.height = '0';
 	el.offsetHeight; // reflow
-	el.style.height = Math.min(elementHeight, props.maxHeight ?? Infinity) + 'px';
+	el.style.height = `${Math.min(elementHeight, props.maxHeight ?? Infinity)}px`;
 }
 
-function afterEnter(el) {
-	el.style.height = null;
+function afterEnter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
-function leave(el) {
+function leave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementHeight = el.getBoundingClientRect().height;
-	el.style.height = elementHeight + 'px';
+	el.style.height = `${elementHeight}px`;
 	el.offsetHeight; // reflow
-	el.style.height = 0;
+	el.style.height = '0';
 }
 
-function afterLeave(el) {
-	el.style.height = null;
+function afterLeave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	el.style.height = '';
 }
 
 function toggle() {
@@ -117,7 +113,7 @@ function toggle() {
 
 onMounted(() => {
 	const computedStyle = getComputedStyle(document.documentElement);
-	const parentBg = getBgColor(rootEl.value!.parentElement!);
+	const parentBg = getBgColor(rootEl.value?.parentElement) ?? 'transparent';
 	const myBg = computedStyle.getPropertyValue('--MI_THEME-panel');
 	bgSame.value = parentBg === myBg;
 });
diff --git a/packages/frontend/src/components/global/MkPageHeader.tabs.vue b/packages/frontend/src/components/global/MkPageHeader.tabs.vue
index adf8638dae..aaef8b8fca 100644
--- a/packages/frontend/src/components/global/MkPageHeader.tabs.vue
+++ b/packages/frontend/src/components/global/MkPageHeader.tabs.vue
@@ -53,7 +53,7 @@ export type Tab = {
 </script>
 
 <script lang="ts" setup>
-import { onMounted, onUnmounted, watch, nextTick, shallowRef } from 'vue';
+import { nextTick, onMounted, onUnmounted, shallowRef, watch } from 'vue';
 import { defaultStore } from '@/store.js';
 
 const props = withDefaults(defineProps<{
@@ -120,14 +120,14 @@ function onTabWheel(ev: WheelEvent) {
 
 let entering = false;
 
-async function enter(element: Element) {
+async function enter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	entering = true;
-	const el = element as HTMLElement;
 	const elementWidth = el.getBoundingClientRect().width;
 	el.style.width = '0';
 	el.style.paddingLeft = '0';
-	el.offsetWidth; // force reflow
-	el.style.width = elementWidth + 'px';
+	el.offsetWidth; // reflow
+	el.style.width = `${elementWidth}px`;
 	el.style.paddingLeft = '';
 	nextTick(() => {
 		entering = false;
@@ -136,22 +136,23 @@ async function enter(element: Element) {
 	setTimeout(renderTab, 170);
 }
 
-function afterEnter(element: Element) {
-	//el.style.width = '';
+function afterEnter(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
+	// element.style.width = '';
 }
 
-async function leave(element: Element) {
-	const el = element as HTMLElement;
+async function leave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	const elementWidth = el.getBoundingClientRect().width;
-	el.style.width = elementWidth + 'px';
+	el.style.width = `${elementWidth}px`;
 	el.style.paddingLeft = '';
-	el.offsetWidth; // force reflow
+	el.offsetWidth; // reflow
 	el.style.width = '0';
 	el.style.paddingLeft = '0';
 }
 
-function afterLeave(element: Element) {
-	const el = element as HTMLElement;
+function afterLeave(el: Element) {
+	if (!(el instanceof HTMLElement)) return;
 	el.style.width = '';
 }
 
diff --git a/packages/frontend/src/directives/adaptive-bg.ts b/packages/frontend/src/directives/adaptive-bg.ts
index 45891de889..f88996019f 100644
--- a/packages/frontend/src/directives/adaptive-bg.ts
+++ b/packages/frontend/src/directives/adaptive-bg.ts
@@ -4,19 +4,11 @@
  */
 
 import { Directive } from 'vue';
+import { getBgColor } from '@/scripts/get-bg-color.js';
 
 export default {
 	mounted(src, binding, vn) {
-		const getBgColor = (el: HTMLElement) => {
-			const style = window.getComputedStyle(el);
-			if (style.backgroundColor && !['rgba(0, 0, 0, 0)', 'rgba(0,0,0,0)', 'transparent'].includes(style.backgroundColor)) {
-				return style.backgroundColor;
-			} else {
-				return el.parentElement ? getBgColor(el.parentElement) : 'transparent';
-			}
-		};
-
-		const parentBg = getBgColor(src.parentElement);
+		const parentBg = getBgColor(src.parentElement) ?? 'transparent';
 
 		const myBg = window.getComputedStyle(src).backgroundColor;
 
diff --git a/packages/frontend/src/directives/adaptive-border.ts b/packages/frontend/src/directives/adaptive-border.ts
index 685ca38e96..1305f312bd 100644
--- a/packages/frontend/src/directives/adaptive-border.ts
+++ b/packages/frontend/src/directives/adaptive-border.ts
@@ -4,19 +4,11 @@
  */
 
 import { Directive } from 'vue';
+import { getBgColor } from '@/scripts/get-bg-color.js';
 
 export default {
 	mounted(src, binding, vn) {
-		const getBgColor = (el: HTMLElement) => {
-			const style = window.getComputedStyle(el);
-			if (style.backgroundColor && !['rgba(0, 0, 0, 0)', 'rgba(0,0,0,0)', 'transparent'].includes(style.backgroundColor)) {
-				return style.backgroundColor;
-			} else {
-				return el.parentElement ? getBgColor(el.parentElement) : 'transparent';
-			}
-		};
-
-		const parentBg = getBgColor(src.parentElement);
+		const parentBg = getBgColor(src.parentElement) ?? 'transparent';
 
 		const myBg = window.getComputedStyle(src).backgroundColor;
 
diff --git a/packages/frontend/src/directives/panel.ts b/packages/frontend/src/directives/panel.ts
index 7b5969c679..aa26b94d0b 100644
--- a/packages/frontend/src/directives/panel.ts
+++ b/packages/frontend/src/directives/panel.ts
@@ -4,19 +4,11 @@
  */
 
 import { Directive } from 'vue';
+import { getBgColor } from '@/scripts/get-bg-color.js';
 
 export default {
 	mounted(src, binding, vn) {
-		const getBgColor = (el: HTMLElement) => {
-			const style = window.getComputedStyle(el);
-			if (style.backgroundColor && !['rgba(0, 0, 0, 0)', 'rgba(0,0,0,0)', 'transparent'].includes(style.backgroundColor)) {
-				return style.backgroundColor;
-			} else {
-				return el.parentElement ? getBgColor(el.parentElement) : 'transparent';
-			}
-		};
-
-		const parentBg = getBgColor(src.parentElement);
+		const parentBg = getBgColor(src.parentElement) ?? 'transparent';
 
 		const myBg = getComputedStyle(document.documentElement).getPropertyValue('--MI_THEME-panel');
 
diff --git a/packages/frontend/src/scripts/get-bg-color.ts b/packages/frontend/src/scripts/get-bg-color.ts
new file mode 100644
index 0000000000..ccf60b454f
--- /dev/null
+++ b/packages/frontend/src/scripts/get-bg-color.ts
@@ -0,0 +1,18 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import tinycolor from 'tinycolor2';
+
+export const getBgColor = (elem?: Element | null | undefined): string | null => {
+	if (elem == null) return null;
+
+	const { backgroundColor: bg } = window.getComputedStyle(elem);
+
+	if (bg && tinycolor(bg).getAlpha() !== 0) {
+		return bg;
+	}
+
+	return getBgColor(elem.parentElement);
+};