tl;dr A normal spinner
I tried to find this out by looking on how this spinner is generated. Fortunately Stack Exchange documented their styles quite well on stackoverflow.design. They developed a framework called Stacks.
This button style is generated by the class is-loading
. At first I looked whether it is an SVG but it uses an other trick.
They generate an pseudo-element (::before
) on the button that contains the following code:
.s-btn.is-loading::before {
content: "";
position: absolute;
top: calc(50% - 8px);
width: 16px;
height: 16px;
margin-left: -1.65em;
margin-left: -22px;
border: 2px dotted rgba(57,115,157,.4);
border-left-color: #33658a;
border-radius: 50%;
animation: spinnerRotate .8s infinite linear;
}
This should generate a rotating circle with equally distributed dots, like in this zoomed in picture:

I just tried the image uploader and it seems like I can't reproduce this problem. Therefore I think it is a bug only ocurring in certain browsers. Maybe consider changing your question to a bug-report or start a new question.
I have put the important code in a snippet. If you execute this snippet you should see the same effect. (If you don't want to try the image uploader)
.loader::before {
content: "";
position: absolute;
top: calc(50% - 8px);
width: 16px;
height: 16px;
margin-left: -1.65em;
margin-left: -22px;
border: 2px dotted rgba(57, 115, 157, .4);
border-left-color: #33658a;
border-radius: 50%;
animation: spinnerRotate .8s infinite linear;
}
.loader {
padding-left: 2.2em;
position: relative;
}
@keyframes spinnerRotate {
from {
transform: rotate(0)
}
to {
transform: rotate(359deg)
}
}
<div class="loader">← Loader</div>