为什么我的Bootstrap网格中的行重叠？

Question

希望有人可以帮助我找出我所缺少的东西。现在，行重叠。我希望根据图像的大小（它们都是相同的大小）适当地间隔开两行。我不知道我错过了什么让它以我想要的方式回应。

Code

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    img {
      max-width: 100%;
      -webkit-backface-visibility: hidden;
    }
    
    .gallery-project-thumb {
      position: relative;
    }
    
    .vert-pad {
        padding-top: 30px;
        padding-bottom: 30px;
    }
    
    .gallery-project-thumb img:first-child {
        border: 8px solid #fff;
        -webkit-box-shadow: 0 0 6px 0.5px rgba(0,0,0,.5);
        -moz-box-shadow: 0 0 6px .5px rgba(0,0,0,.5);
        box-shadow: 0 0 6px 0.5px rgba(0,0,0,.5);
        position: absolute;
        z-index: 3;
        -webkit-backface-visibility: hidden;
        display: block;
    }
    
    .gallery-project-thumb img:nth-child(2) {
        z-index: 2;
        margin-bottom: 15px;
        border: 8px solid #fff;
        -webkit-box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        -moz-box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        position: absolute;
        -ms-transform: rotate(-2deg);
        -webkit-transform: rotate(-2deg);
        transform: rotate(-2deg);
        -webkit-backface-visibility: hidden;
    }
    .gallery-project-thumb img:nth-child(3) {
        z-index: 1;
        margin-bottom: 15px;
        border: 8px solid #fff;
        -webkit-box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        -moz-box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        box-shadow: 0 0 6px 1px rgba(0,0,0,.5);
        position: absolute;
        -ms-transform: rotate(2deg);
        -webkit-transform: rotate(2deg);
        transform: rotate(2deg);
        -webkit-backface-visibility: hidden;
    }

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<div class="row vert-pad gallery-project-container" style="margin-left: 75px; margin-right: 20px;">
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    </div>
    <div class="row vert-pad gallery-project-container" style="margin-left: 75px; margin-right: 20px;">
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    
    <div class="col-sm-6 col-md-4 col-lg-3">
    <div class="gallery-project-thumb"><img src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"> <img class="gallery-project-thumb-background" src="https://via.placeholder.com/840x560"></div>
    </div>
    </div>

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Answer 1

由于图像处于绝对位置，因此浏览器不知道制作这些元素的高度。如果您将.gallery-project-thumb类的高度设置为应设置的图像高度

.gallery-project-thumb{
    height: 300px;
}