www programming

ah ok nice i see ... the first, given by wordpress i assume, with these little animals rounding work fine because on css, i assume the preloader is "styled" to scroll when page scroll, while the second little that display in phpBB, and say Loading ... is styled via css (or css output by javascript maybe) in a way that not scroll (or remain centered) when page scroll. Think you need to start checking this. You know how/OR have you take a look into this?

sorry i've re-read now, it is on wp, also the little second one, but the reason is the same, the css i assume

Really got no clue how to look into this, since it's an extension for phpbb.

this one https://www.phpbb.com/customise/db/exte ... _indicator

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and yeah, the ..loading one, scrolls with the page on the phpbb forum, but not in the iframed forum.
and just found out that it's not a "preload indicator" but a loading indicator

... in the while check this also:
into the overall_footer.html code you've add
there is this line: but it need to be: like also result into your page-forum.php output code,
where line: these two lines need to be the same on both overall_footer.html and page-forum.php code

here the loader in phpBB is styled:
view-source:http://rng.clanroyal.dk/forums/ext/dmzx ... version=41

so check the file:
/forums/ext/dmzx/loadingindicator/styles/prosilver/theme/loading_indicator.css
this is the file that style the html code that output into the phpBB page, which is:
<div id="loading-indicator">
<span>Loading...</span>
</div>
let know if u can get out of this!

[EDITED]

p.s and here:
http://rng.clanroyal.dk/forums/ext/dmzx ... version=41
when the Loader display, it is styled (and fired) by just this line in js, that push the inline style: so the loader display: you should add the style via this js function to the element, not only 'display:block' css OR, change/switch the class within this js code into the element, styling via css.
Hope it is clear what i mean.

This also can be a way to resolve your issue. Or as said above, change css declarations on the prev post linked css file.