So this is given by 2 things you need to consider (and you may already imagine)
1) the wordpress page
2) the phpBB page
in iframe mode you'll have to check the wordpress wrapper, padding or margin, AND the same in phpBB.
The red background into image you attached, seem to come from wordpress, is it right?
If it is, you need to change the css that cause this gap in wordpress.
Possible solution is to edit not the page-forum.php (or whatever you named it, into your active template folder)
where there is this code
Code: Select all
<!-- START iframe div -->
<div id="w3_toogle_wrap_loader" class="w3_no_wrap_loader"><div class="w3_loader"></div></div>
<div class="">
you could add a class to the div that contain the iframe or even inline css code
Code: Select all
<div class="myclass" style="padding:0px;margin:0px;width:100%">
but this may will not solve.
It depend by elements that are around, added (maybe) by your wp theme.
In fact this page
https://www.csct38.com/amarrages/
looks like to have the same gap for the central element inside