A lazy chunk is a chunk nothing tests
The code that only loads when somebody clicks is the code your entire test suite is blind to.
A deploy removed three JavaScript files that nothing on any page referenced until a control was clicked. Types were green. The build was green. Health checks were green. Screenshots were identical. The first person to click got an error boundary.
Lazily loaded code is invisible to anything that only loads pages. The reachability question is not what does the page ask for, it is what does the code the page asked for ask for NEXT, and that answer lives inside the bundles themselves.
Walk it the way a browser would, from every file actually on disk, and fail on any file they name that is not there.