Setting breakpoints in Sapper server code -- is it feasible?

I was playing around with a fresh Sapper template ("sveltejs/sapper-template#webpack") and wanted to set a breakpoint in src/server.js using WebStorm. Setting the breakpoint is easy enough, but I'm unable to get the debugger to suspend execution when hitting the breakpoint. I tried a bunch of different things before it dawned on me that there's probably no way to get this to work due to how Svelte/Sapper works, i.e. the compiler generates a set of new files which is what is actually executed. So when I set a breakpoint in the "developer-facing" file that has zero effect since it's the generated files that are actually executed. Specifically I'm talking about the developer-facing file src/server.js file and the generated file in __sapper__/dev/server/server.js.

I came across a YouTube video where what I'm trying to do is demonstrated successfully: In the video the author uses the Sapper cli.js as the driver and dev as the argument -- this is what happens under the hood when you run npm run dev. However, as you'll see in the video, the server.js file that is successfully being used is the one in src/ and not the generated one in __sapper__/build/server/. This does not make sense to me.

So, my questions are:

1) Has anyone else been able to set breakpoints on developer-facing Sapper user code like what I'm describing above and what is shown in the video? If so, how did you do it?

2) If setting breakpoints directly like this is not possible, is it feasible to use the generated code instead, but make some changes to the build step so that the generated code is more suitable for debugging?

If there are other options I'm also interested in hearing about those. Thanks!