This happens because you use position:relative on the #previous and #next elements. Like this they are repositioned but still use up the space they would originally occupy.
Use the following css instead:
.block-wapper {
    position:relative;
    ...
}
#previous {
    position: absolute;
    left: 10px;
    ...
}
#next {
    position: absolute;
    right: 10px;
    ...
}
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solved How to get rid of hidden space HTML/CSS [closed]