Browser: Opera 12.00 beta versions (build 1441, others might be affected); Midori 0.4.1
Operating Systems: Linux so far but nothing indicates other OSes might be unaffected.
When visiting dokuwiki.org as an guest the site behaves as expected and shows the old default template. No problems there.
After logging in via the wiki login page, however, template is switched to the "new default template" of DokuWiki and dokuwiki.org. This is where problems begin. When first visiting pages, they still work well.
After switching to another tab or window however and then returning to a tab featuring logged-in dokuwiki.org, browser instantly becomes largely unresponsive. Disk usage and memory consumption by the browser skyrocket and it is not possible to switch to any other tab nor to activate any link or button element. Eventually disk swap space starts getting filled, causing the entire system to become unresponsive. After the swap space is almost filled 100% disk usage doesn't stop (HDD led permanently turned on, heavy HDD noise), and can continue for several minutes until it either a) somehow cleans itself by dumping most of the content to RAM or b) the entire system deadlocks. If the system somehow manages to resume responsive state, however, any attempt to interact with the logged in dokuwiki.org tab will trigger the problem again.
In the case of Opera it is not possible to access eg.: the error console because no controls will respond, so it is not possible for me to find out any error message about this.
This only happens with the dokuwiki.org site, I have tried various configurations (such as 80+ low-load tabs or 20+ heavy-load tabs) and it's always the tab with the new template to trigger this issue.
Tagged with severity High, not Critical, because this behaviour is detectable (and thus browser processess can be killed) before full system deadlock kicks in.