Currently, the same root path is used for filesystem paths as well as for urls and assets. URL and asset URLs are determined relative to the URL in the address bar, whereas filesytem paths are relative to the actual file being executed.
When attempting to use "pretty urls" or an alternate form of routing with path info, this causes CSS and assets to break (i.e. not be loaded) because the URL pointing to it is not correct. To clarify, the app.php file in the root of the phpBB installation is called, with a phpbb_root_path of './', but the URL might be domain.com/phpBB/app.php/my/route. The filesystem path is correct, but the URL path is relative to ./app.php/my/route which does not exist.
As such, we need to separate the path used for assets and CSS and such from filesystem paths OR we need to automatically correct paths at run time based on the URL.
NOTE: This ticket is required for the Controller system to properly use path info for routing. Until then, it will need to use the query string (i.e. "app.php?controller=my/route" instead of "app.php/my/route" or, with rewriting, "my/route").