*mutation_summary.js* may be an exception, since we got it from elsewhere, and it's third-party code, but let's use a hyphen instead of an underscore to separate words in the names of our own files, i.e. call this window-manager.js. (I'd prefer to do this even for mutation_summary.js, i.e. call it mutation-summary.js, especially since we're copying that file rather than using it from a submodule, so renaming the file doesn't fork it.)