How to debug Latte templates on PhpStorm?
What I usually do when debugging templates or other generated code is setting a breakpoint at a place where the template is generated and then stepping inside until I get inside the generated file. Or you can use a breakpoint in a known method (inside a helper for example) and then step out until you're in the template.
I'm afraid that there is no chance to set up latte debugging at least up to the point where standardized sourcemaps are a thing :)