When under DIRACOS,
self.root = rootPath in src/WebAppDIRAC/Core/StaticHandler.py is set to diracos directory.
In case source repository is outside of that directory (f.e. in parallel directory), tornado will not pickup static files (since they are "outside root").
WebApp/Routing VERBOSE: Static paths found knows all static directories in use, so I think self.root should point to common directory for all of them (in case that sounds insecure, it probably can be dynamic).