So you have a positioning problem which can be fixed with the CSS z-index
property.
You can set the header
to have a high z-index
which would make it show above everything unless you set an element to have a higher z-index
.
.header {
z-index: 999;
}
In your case I think the z-index on the header would be best (I've targeted the class .header as if you were to have more elements you wouldn't want them all to be at a high z-index most likely)
You can view more about z-index
values at MDN
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