Special:EventDetails uses a tabbed layout implemented in JavaScript. When switching tabs, the transition happens entirely in JavaScript, and there is no trace of it in the URL or elsewhere. The only way to get a direct link to the tab you're on is to right click on the tab header and copy its link. I think it would be nice to automatically update the URL with the tab currently open. A couple things to keep in mind/consider:
- This would likely append to the history, not replace, so that users can navigate to the previous tab with the "back" arrow
- If there's already a tab parameter in the URL, we would obviously update that instead of adding a duplicate
- If there is no tab parameter and the "event details" tab is currently open (i.e., default state when visiting the page), do not add a tab parameter.