CSS: position:fixed inside of position:absolute

You mentioned in the comments that OpenLayers uses CSS transforms. That being the case:

the element with fixed positioning will become relative to the element with the transform – not relative to the viewport

Take a look at the spec: The Transform Rendering Model

Specifying a value other than ‘none’ for the ‘transform’ property
establishes a new local coordinate system at the element that it is
applied to.

.wpr
{
    width: 200px;
    height:1000px;
    background: pink;
    position:relative;
    margin: 0 200px;
    -webkit-transform: translateX(0);
    transform: translateX(0);
}
.fixed
{
    width: 200px;
    height:200px;
    margin: 50px;
    position: fixed;    
    top:0;
    left:0;
    background: aqua;
}

<div class="wpr">
    <div class="fixed"></div>
</div>