交叉口观察者不能使用位置：固定的目标 [英] Intersection observer does not work with target with position: fixed
问题描述
我试图通过交集观察者调用回调。
I am trying to invoke a callback via intersection observer.
我希望目标为样式：position：fixed并通过
style.top 移动它。
I want the target to be style: "position: fixed" and move it via
style.top.
我还用 style指定了根元素，它是目标的祖先：position：relative。
但是当目标和观察者相交时，回调函数不会被触发。
But when the target and the observer intersects, the callback function won't be triggered.
是否有我错过了一些限制？
Are there some limitations I missed?
这是我输入的内容：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>IO</title>
</head>
<body>
<div style="height: 200px;width: 100%;background: violet" class="upper">aaa</div>
<div style="position:relative;height: 200px;width: 100%;background: blueviolet" id="middle">bbb
<div id="target" style="position:fixed;top: 0px;width: 50px;height: 50px;background: firebrick">ccc</div>
</div>
<script>
let options = {
root: document.getElementById("middle"),
rootMargin: '0px',
threshold: 0
};
let observer = new IntersectionObserver(entry => {
console.log("observer's acting.")
}, options);
let target = document.getElementById("target");
observer.observe(target);
let stepping = 0;
let cb = () => {
target.style.top = stepping + 'px';
stepping += 4;
if (stepping < 300){
setTimeout(cb, 100);
}
};
window.addEventListener("click", () => {
cb();
})
</script>
</body>
</html>
这是一个codepen演示：
codepen demo
And here is a codepen demo: codepen demo
您可以点击页面中的任意位置开始移动 ccc 阻止。
You can click anywhere in the page to start moving the ccc block.
推荐答案
元素 position：fixed 相对于视口定位，视口移动。因此，当您滚动时，固定定位元素会移动。即使 #target 是 #middle 的孩子，我相信IntersectionObserver，无论它在引擎盖下使用什么计算目标是否进入/离开 root ，从不触发回调，因为目标在文档流程之外。
Elements with position: fixed are positioned relative to the viewport and the viewport moves. So, fixed positioned elements "move" as you scroll. Even though #target is a child of #middle, I believe the IntersectionObserver, with whatever it uses under the hood to calculate if the target is entering/leaving the root, never fires the callback because the target is outside of the document flow.
这是一个相关问题。与此问题相关的互联网中没有太多内容： https： //bugs.chromium.org/p/chromium/issues/detail?id=653240
Here is a related issue. There isn't much out in the interwebs related to this issue: https://bugs.chromium.org/p/chromium/issues/detail?id=653240
注意：设置 position：absolute目标确实在进入和离开视口时触发了回调。
Note: Setting position: absolute on the target does indeed fire the callback when entering and leaving the viewport.
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