Почему это не будет работать с CSS
Если вы хотите избежать отрицательных полей или иметь дело с переменной высотой, то это не будет работать с CSS.Ваш относительный контейнер #sticky-container имеет содержимое #full-height-content с высотой 400px.Позиция sticky не может работать в обоих направлениях.Вы можете либо поместить его в его предшественника прокрутки (документ), либо установив свойство переполнения на #sticky-container, вы можете расположить его там.
Элемент позиционируется в соответствии с обычным потоком документа, а затем смещается относительно ближайшего предка прокрутки и содержащего блока (ближайшего предка уровня блока)
Если вы установите свойство переполнения (скажем, overflow-y: hidden) на #sticky-container, то ваш элемент #sticky-content потенциально будет на 400px вниз в документе, а не на 20px от нижней части области просмотра (что вам нужно).
Вы, похоже, хотите, чтобы липкий элемент прилипал к области просмотра, а затем каким-то образом "прилипал" к элементу #sticky-container, когда #bottom становился видимым.
Как может решить JavaScriptthis
Используя JavaScript и IntersectionObserver, вы можете получить его обоими способами.#sticky-content будет вести себя так, как у вас, пока #bottom не войдет в область просмотра.Как только это произойдет, мы добавим класс (.is-absolute) к #sticky-content, который позиционирует его absolute, а не sticky, и тогда он будет 20px от основания его относительного родителя #sticky-container (и больше не будетего прокручивающему предку, окну просмотра).IntersectionObserver будет обрабатывать обратное удаление этого класса, когда #bottom покидает область просмотра.
Обратите внимание: IntersectionObserver является довольно новым и пока широко не поддерживается всеми основными браузерами.Вам может повезти с polyfill .
// The elements we want to control
var stickyContent = document.querySelector('#sticky-content');
var stickyContentActiveClass = 'is-absolute';
var bottom = document.querySelector('#bottom');
// The function to call when #bottom intersects with the viewport
var callback = function(entries, observer) {
entries.forEach(entry => {
if (entry.isIntersecting) {
stickyContent.classList.add(stickyContentActiveClass);
} else {
stickyContent.classList.remove(stickyContentActiveClass);
}
});
};
// Setting up the observer with an argument of 0 for the threshold
// Our callback function will run as soon as #bottom enters the viewport
var observer = new IntersectionObserver(callback, {threshold: 0});
// Tell our observer to watch #bottom
observer.observe(bottom);
#root-container {
display: flex;
}
#container1 {
width: 100px;
height: 500px;
background-color: grey;
}
#sticky-container {
width: 320px;
max-height: 500px;
position: relative;
background-color: lightgrey;
}
#sticky-container-header {
width: 320px;
height: 100px;
background-color: #2f4d92;
}
#full-height-content {
width: 100%;
height: 400px;
overflow-y: scroll;
background-color: #d67e23;
}
#sticky-content {
width: 80%;
height: 100px;
position: sticky;
bottom: 20px;
background-color: rgba(0,0,0,0.5);
}
#sticky-content.is-absolute {
position: absolute;
}
#bottom {
width: 420px;
height: 100px;
background-color: purple;
}
<div id='root-container'>
<div id="container1"></div>
<div id="sticky-container">
<div id='sticky-container-header'></div>
<div id='full-height-content'>
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</div>
<div id='sticky-content'></div>
</div>
</div>
<div id="bottom">
</div>