To repro:

1. Launch an Electron app (hello world/default fiddle will do)
2. Open DevTools
3. Find a minified JS file, e.g. renderer_init.js
4. Click the "Pretty Print" button in DevTools

Expected behavior:
The JS in the DevTools view is replaced with a pretty-printed version of the code

Actual behavior:
The JS in the DevTools view is unchanged


Bisected to https://github.com/electron/electron/compare/v11.0.0-nightly.20200721...v11.0.0-nightly.20200723
shell/browser/ui/devtools_ui.cc
return GetMimeTypeForPath(path);
  }
  bool ShouldAddContentSecurityPolicy() override { return false; }
  bool ShouldDenyXFrameOptions() override { return false; }
  bool ShouldServeMimeTypeAsContentTypeHeader() override { return true; }
  void StartBundledDataRequest(const std::string& path,
                               GotDataCallback callback) {
    std::string filename = PathWithoutParams(path);
    scoped_refptr<base::RefCountedMemory> bytes =
        content::DevToolsFrontendHost::GetFrontendResourceBytes(filename);
    DLOG_IF(WARNING, !bytes)
        << "Unable to find dev tool resource: " << filename
        << ". If you compiled with debug_devtools=1, try running with "
           "--debug-devtools.";
    std::move(callback).Run(bytes);
  }
 private:
  DISALLOW_COPY_AND_ASSIGN(BundledDataSource);
};
}  
DevToolsUI::DevToolsUI(content::BrowserContext* browser_context,
                       content::WebUI* web_ui)
    : WebUIController(web_ui) {
  web_ui->SetBindings(0);
  content::URLDataSource::Add(browser_context,
                              std::make_unique<BundledDataSource>());
}
}