charging-cabinet/software/h5-uniapp/.agents/skills/vue-best-practices/reference/transition-animate-transform-opacity.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Prefer Animating Transform and Opacity for Performance	MEDIUM	Animating layout-triggering properties like height or margin causes expensive reflows and janky animations	best-practice	vue3 transition animation performance css transform opacity

Prefer Animating Transform and Opacity for Performance

Impact: MEDIUM - When creating Vue transitions, you should prefer animating transform and opacity properties. These are GPU-accelerated and do not trigger expensive CSS layout recalculations. Animating properties like height, width, margin, padding, or top/left causes the browser to recalculate layout on every frame, resulting in janky, low-performance animations.

Task Checklist

• Use transform for position and size animations (translate, scale, rotate)
• Use opacity for fade effects
• Avoid animating height, width, margin, padding, top, left, right, bottom
• If you must animate height, consider using max-height with a generous value or JavaScript-based solutions
• Use will-change sparingly and only when needed

Problematic Code:

/* BAD: Animating height triggers layout recalculation every frame */
.slide-enter-active,
.slide-leave-active {
  transition: height 0.3s ease;
}

.slide-enter-from,
.slide-leave-to {
  height: 0;
}

.slide-enter-to,
.slide-leave-from {
  height: 200px;
}

/* BAD: Animating margin causes layout thrashing */
.slide-enter-active,
.slide-leave-active {
  transition: margin-top 0.3s ease;
}

.slide-enter-from,
.slide-leave-to {
  margin-top: -100%;
}

Correct Code:

/* GOOD: Using transform and opacity - GPU accelerated */
.fade-enter-active,
.fade-leave-active {
  transition: opacity 0.3s ease;
}

.fade-enter-from,
.fade-leave-to {
  opacity: 0;
}

/* GOOD: Using transform for slide animations */
.slide-enter-active,
.slide-leave-active {
  transition: transform 0.3s ease, opacity 0.3s ease;
}

.slide-enter-from {
  transform: translateX(-100%);
  opacity: 0;
}

.slide-leave-to {
  transform: translateX(100%);
  opacity: 0;
}

/* GOOD: Using scale instead of width/height */
.scale-enter-active,
.scale-leave-active {
  transition: transform 0.3s ease, opacity 0.3s ease;
}

.scale-enter-from,
.scale-leave-to {
  transform: scale(0);
  opacity: 0;
}

When You Must Animate Height

If you absolutely need to animate height (e.g., accordion), consider these alternatives:

<template>
  <!-- Option 1: max-height trick (simple but not precise) -->
  <Transition name="expand">
    <div v-if="expanded" class="content">
      <slot />
    </div>
  </Transition>
</template>

<style>
.expand-enter-active,
.expand-leave-active {
  transition: max-height 0.3s ease, opacity 0.3s ease;
  overflow: hidden;
}

.expand-enter-from,
.expand-leave-to {
  max-height: 0;
  opacity: 0;
}

.expand-enter-to,
.expand-leave-from {
  max-height: 500px; /* Set to expected max content height */
  opacity: 1;
}
</style>

<script setup>
// Option 2: JavaScript hooks for precise height animation
function onEnter(el, done) {
  el.style.height = '0'
  el.style.overflow = 'hidden'
  // Force reflow
  el.offsetHeight
  el.style.transition = 'height 0.3s ease'
  el.style.height = el.scrollHeight + 'px'

  el.addEventListener('transitionend', () => {
    el.style.height = ''
    el.style.overflow = ''
    done()
  }, { once: true })
}

function onLeave(el, done) {
  el.style.height = el.scrollHeight + 'px'
  el.style.overflow = 'hidden'
  el.offsetHeight
  el.style.transition = 'height 0.3s ease'
  el.style.height = '0'

  el.addEventListener('transitionend', done, { once: true })
}
</script>

<template>
  <Transition @enter="onEnter" @leave="onLeave">
    <div v-if="expanded">
      <slot />
    </div>
  </Transition>
</template>

Performance Comparison

Property	Layout	Paint	Composite	Performance
transform	No	No	Yes	Excellent
opacity	No	No	Yes	Excellent
background-color	No	Yes	Yes	Good
width/height	Yes	Yes	Yes	Poor
margin/padding	Yes	Yes	Yes	Poor
top/left	Yes	Yes	Yes	Poor

Reference

• Vue.js Transition Documentation
• CSS Triggers - Reference for which properties trigger layout/paint
• High Performance Animations