ee = document.getElementById("test_tree");
        let el_source = CodeMirror(document.getElementById("source"), {
            lineNumbers: true,
            theme: "seti",
            mode: "text/x-c++src",
            readOnly: true,
        });

        addEventListener('beforeunload', () => {
            fetch("viewer.closed");
        });

        window.onload = function () {
            el_source.doc.setValue("// Loading... ");
            fetch("coverage.dat").then(response =>
                respon