perf(frontend): introduce MkLazy for lazy loading

packages/frontend/src/components/global/MkLazy.vue

@@ -0,0 +1,53 @@
+<!--
+SPDX-FileCopyrightText: syuilo and other misskey contributors
+SPDX-License-Identifier: AGPL-3.0-only
+-->
+
+<template>
+<div ref="rootEl" :class="$style.root">
+	<div v-if="!showing" :class="$style.placeholder"></div>
+	<slot v-else></slot>
+</div>
+</template>
+
+<script lang="ts" setup>
+import { nextTick, onMounted, onActivated, onBeforeUnmount, ref, shallowRef } from 'vue';
+
+const rootEl = shallowRef<HTMLDivElement>();
+const showing = ref(false);
+
+const observer = new IntersectionObserver(
+	(entries) => {
+		if (entries.some((entry) => entry.isIntersecting)) {
+			showing.value = true;
+		}
+	},
+);
+
+onMounted(() => {
+	nextTick(() => {
+		observer.observe(rootEl.value!);
+	});
+});
+
+onActivated(() => {
+	nextTick(() => {
+		observer.observe(rootEl.value!);
+	});
+});
+
+onBeforeUnmount(() => {
+	observer.disconnect();
+});
+</script>
+
+<style lang="scss" module>
+.root {
+	display: block;
+}
+
+.placeholder {
+	display: block;
+	min-height: 150px;
+}
+</style>

packages/frontend/src/components/index.ts

@@ -25,6 +25,7 @@ import MkPageHeader from './global/MkPageHeader.vue';
 import MkSpacer from './global/MkSpacer.vue';
 import MkFooterSpacer from './global/MkFooterSpacer.vue';
 import MkStickyContainer from './global/MkStickyContainer.vue';
+import MkLazy from './global/MkLazy.vue';
 
 export default function(app: App) {
 	for (const [key, value] of Object.entries(components)) {
@@ -53,6 +54,7 @@ export const components = {
 	MkSpacer: MkSpacer,
 	MkFooterSpacer: MkFooterSpacer,
 	MkStickyContainer: MkStickyContainer,
+	MkLazy: MkLazy,
 };
 
 declare module '@vue/runtime-core' {
@@ -77,5 +79,6 @@ declare module '@vue/runtime-core' {
 		MkSpacer: typeof MkSpacer;
 		MkFooterSpacer: typeof MkFooterSpacer;
 		MkStickyContainer: typeof MkStickyContainer;
+		MkLazy: typeof MkLazy;
 	}
 }