Short Answer
The problem is that Safari is not recognizing the height: 100% on the img elements.
Explanation
This is not a Safari bug. It’s just a different interpretation of the spec.
When dealing with percentage heights, some browsers (like Safari) adhere to the traditional interpretation of the spec, which requires a defined height on the parent.
10.5 Content height: the
height
property
<percentage>Specifies a percentage height. The percentage is calculated with
respect to the height of the generated box’s containing block. If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the used height is calculated as ifautowas specified.
In other words, a percentage height on an in-flow element will be recognized only when the parent has a defined height.
Some browsers, such as Chrome and Firefox, have moved past this interpretation and now accept flex and grid heights as an adequate parent reference for a child with a percentage height.
But Safari is stuck in the past. This doesn’t mean it’s wrong, invalid or a bug.
The last substantive update to the CSS height definition was in 1998 (CSS2). With so many new CSS properties and technologies since that time, the definition has become obsolete, unclear and woefully incomplete. Until the definition is updated for modern use, browser rendering variations can be expected.
Solution
Since Safari doesn’t recognize the height: 100% on the img elements, and you can’t specify a height on the parent (#thumbnailContainer) because that height is defined by grid-template-rows: 1fr on the top-level container, you can try using flexbox.
By making #thumbnailContainer a flex container, you can define the size of the images (flex items) using flex properties.
#grid {
height: 100vh;
display: grid;
grid-template-rows: 1fr 1fr 1fr;
}
#thumbnailContainer {
display: flex;
overflow-x: auto;
overflow-y: hidden;
min-width: 0;
min-height: 0;
}
img {
flex: 0 0 35%;
min-width: 0;
object-fit: cover;
}
header, footer {
background-color: dodgerblue;
}<div id="grid">
<header>Header</header>
<div id="thumbnailContainer">
<img src="https://c2.staticflickr.com/8/7591/16903911106_b7ced9d758.jpg">
<img src="https://c1.staticflickr.com/9/8740/16927517701_810fcb2a7c.jpg">
<img src="https://c2.staticflickr.com/8/7637/16902583636_15138a68f0.jpg">
<img src="https://c2.staticflickr.com/8/7614/16927530091_6755845b13.jpg">
<img src="https://c1.staticflickr.com/9/8700/16741099010_d0ecd9df1f.jpg">
<img src="https://c1.staticflickr.com/9/8745/16927567841_74fd20d01d.jpg">
</div>
<footer>Footer</footer>
</div>