js 动态设置元素fixed 页面抖动问题？

Question

windows监听滚动事件，滚动满足fixed条件时，元素一直抖动是什么原因？

window.addEventListener('scroll', this.handleTabFix, true)


handleTabFix () {
  let timeOut = null
  clearTimeout(timeOut)
  timeOut = setTimeout(() => {
    let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
    let offsetTop = document.querySelector('#testNavBar') && document.querySelector('#testNavBar').offsetTop + 60
    scrollTop > offsetTop ? this.isFixTab = true : this.isFixTab = false
    // isFixTab为true时，设置元素为fixed
  }, 1000)
}

Answer 1

猜测是你设置某个元素 fixed 之后，页面高度就不够了；然后它就又被解除 fixed，然后页面高度又超了；于是反复。

解决方案，有两个：

1. 用 position: sticky
2. 给这个元素套一个壳，固定高度，fixed 之后留着壳撑页面

Answer 2

let timeOut = null;

window.addEventListener('scroll', () => {
  if (timeOut) {
    cancelAnimationFrame(timeOut);
  }
  timeOut = requestAnimationFrame(handleTabFix);
}, true);

function handleTabFix() {
  let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
  let offsetTop = document.querySelector('#testNavBar') && document.querySelector('#testNavBar').offsetTop + 60;
  
  if (scrollTop > offsetTop) {
    if (!document.querySelector('.placeholder')) {
      let placeholder = document.createElement('div');
      placeholder.style.height = '60px'; 
      placeholder.className = 'placeholder';
      document.querySelector('#testNavBar').insertAdjacentElement('beforebegin',    placeholder);
}
this.isFixTab = true;
} else {
if (document.querySelector('.placeholder')) {
document.querySelector('.placeholder').remove();
}
this.isFixTab = false;
}
}