box-flex

box-flex.css

.main {
    display: -moz-box; /* Firefox */
    display: -webkit-box; /* Safari and Chrome */
    display: box;
    width: 100%;
    height: 40rem;
}

.main>nav {
    display: -moz-box; /* Firefox */
    display: -webkit-box; /* Safari and Chrome */
    display: box;
    min-width: 4rem;
    border: 1px solid green;
}

.main>.todo-list {
    -moz-box-flex: 2.0; /* Firefox */
    -webkit-box-flex: 2.0; /* Safari and Chrome */
    box-flex: 2.0;
    border: 1px solid blue;
}