SOURCE

<div class="container">
    <div class="box1"></div>
    <div class="box2">1111111111111111111111111111111111111111</div>
</div>

.container {
    width: 100%;
    height: 100px;
    display: flex;
    background: yellow;
    padding: 10px;
}
.box1 {
    flex: 1;
    background: red;
    margin-right: 10px;
}
.box2 {
    width: 40%;
    background: orange;
    min-width: 400px;
    /* overflow: hidden; */
}