Sometimes, looking at unit test output and source code isn't enough to figure out a problem that you're experiencing. Instead, you need to see what's going on as you interact with the application itself. In the browser, you install React tooling that makes it easy to inspect React components as they are related to rendered HTML content.
React also has some built-in performance monitoring capabilities that extend the abilities of the browser developer tools. You can use them to examine and profile your components at a low level.