悬停效果一直保持到光标移动

如果你想让fadeOut动画持续500毫秒，你可以使用内部完整回调：强制重新绘制UI

$(this).hide().show(0);

传递相关上下文：

list.slice(showing - numToShow, numInList).fadeOut(500, onFadeComplete.bind(this));

jQuery(document).ready(function($) {
  var list = $(".partners__wrap .partner");
  var numToShow = 1;
  var button = $(".partners__button__a");
  var numInList = list.length;
  var isShowing = true;
  list.hide();
  if (numInList > numToShow) {
    button.show();
  }
  list.slice(0, numToShow).show();
  button.click(function() {
    var showing = list.filter(':visible').length;
    if (isShowing) {
      list.slice(showing - 1, showing + numToShow).fadeIn(500);
      onFadeComplete();
    } else {
      list.slice(showing - numToShow, numInList).fadeOut(500, onFadeComplete.bind(this));
    }
  });
  function onFadeComplete() {
    $(this).hide().show(0);
    var nowShowing = list.filter(':visible').length;
    if (nowShowing == numInList && isShowing) {
      isShowing = false;
      button.text("Show less");
    } else if (isShowing) {
      button.text("Show even more");
    }
    if (nowShowing == numToShow) {
      button.text("See other");
      isShowing = true;
    }
  }
});

.partners__button {
  text-align: center;
  padding-top: 16px;
}
.partners__button__a {
  -webkit-border-radius: 28px;
  -moz-border-radius: 28px;
  border-radius: 28px;
  font-family: Arial;
  color: #ff4e50;
  font-size: 14px;
  background: #fff;
  padding: 10px 26px 10px 26px;
  text-decoration: none;
  border: 2px solid #ff4e50;
  font-weight: bold;
  outline: none;
  cursor: pointer;
}
.partners__button__a:hover {
  background-color: #3cb0fd;
  border-color: #3cb0fd;
  color: #fff;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="partners">
  <h2 class="partners__h2" id="find">Headline</h2>
  <div class="partners__wrap">
    <div class="partner__1 partner">
      <div class="partner__pic__wrap">
        <div class="partner__1__icon partner__icon"></div>
        <a class="partner__1__pic partner__pic__hover" href="#"></a>
      </div>
      <div class="partner__wrap">
        <h4 class="partner__name">Title</h4>
        <p class="partner__description">Text. And Text.</p>
      </div>
    </div>
    <div class="partner__2 partner">
      <div class="partner__pic__wrap">
        <div class="partner__2__icon partner__icon"></div>
        <a class="partner__2__pic partner__pic__hover" href="#"></a>
      </div>
      <div class="partner__wrap">
        <h4 class="partner__name">Title</h4>
        <p class="partner__description">Text. And Text.</p>
      </div>
    </div>
    <div class="partner__3 partner">
      <div class="partner__pic__wrap">
        <div class="partner__3__icon partner__icon"></div>
        <a class="partner__3__pic partner__pic__hover" href="#"></a>
      </div>
      <div class="partner_wrap">
        <h4 class="partner__name">Title</h4>
        <p class="partner__description">Text. And Text.</p>
      </div>
    </div>
  </div>
</div>
<div class="partners__button">
  <button class="partners__button__a">See other</button>
</div>

这是因为这里的淡出效果

list.slice(showing - numToShow, numInList).fadeOut(500, onFadeComplete);

如果你把500改成0

list.slice(showing - numToShow, numInList).fadeOut(0, onFadeComplete);

这种影响消失了。我不知道fadeOut函数内部发生了什么，但悬停在那里时出现了一些错误。