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>