The .forum-content table has margin: 0 auto in CSS, but the template injects style="max-width:1600px" inline. On load, the browser first centers the table at 1600px, then the CSS width:100% !important and max-width:100% !important snap it to full width. That horizontal recalculation is the layout shift. The inline style and CSS need to agree before first paint.

The Stovebolt Geek
https://www.stovebolt.com/ubbthreads/ubbthreads.php

Server Information
UBB.threads Version 8.0.0
Release 20240826
Server OS Linux
Server Load 0.11
Web Server Apache/2.4.37
PHP Version 8.3.11
MYSQL Version 8.0.39
Database Size 1.82 GB