@Nunomaio afaik you need the z-index. I looked into your code and it seems that the text is a z-index: 0, so your buttons all need at least z-index: 1.
Your code should look like this:
div.relative {
position: fixed;
left: 0px;
z-index: 1 /*at least 1 if it's still glitching try various values*/;
}
Hope this helps,
Johannes
PS: Another way would be to lower the z-index of the text but then you probably need to change the z-index from the images too.