.resizer {
    display: flex;
    margin: 0;
    padding: 0;
    resize: both;
    overflow: hidden
}

    .resizer > .resized {
        flex-grow: 1;
        margin: 0;
        padding: 0;
        border: 0
    }

.ugly {
    background: red;
    border: 4px dashed black;
}