html {
    box-sizing: border-box;
    overflow-y: scroll;
}

*,
*:before,
*:after {
    box-sizing: inherit;
}

#sidebar {
    position: fixed;
    width: 150px;
    height: 100%;
    padding: 20px;
    overflow: hidden;
    background-color: #f5f5f5;
    border-right: 1px solid #eee;
}

#windows {
    float: left;
    margin-left: 150px;
    width: calc(100% - 150px);
    height: 100%;
    padding: 20px 0 0 20px;
}

#contacts {
    list-style-type: none;
}

.contact {
    white-space: nowrap;
}

.window {
    float: left;
    margin: 0 20px 20px 0;
}

.messages {
    height: calc(8.5*1.5em + 2*5px);
    overflow-y: scroll;
    overflow-wrap: break-word;
    border: 1px solid rgb(238, 238, 238);
    border-right: 0;
    margin-bottom: 20px;
    padding: 5px;
}

.message {
    margin-bottom: 0;
    list-style-type: none;
}

.window .input-send {
    width: 100%;
    border: 1px solid rgb(238, 238, 238);
    padding: 0;
    margin-bottom: 0;
}

.close {
    float: right;
    font-weight: bold;
    color: #000000;
    text-shadow: 0 1px 0 #ffffff;
    opacity: 0.2;
    filter: alpha(opacity=20);
    text-decoration: none;
}

.close:hover {
    color: #000000;
    text-decoration: none;
    opacity: 0.4;
    filter: alpha(opacity=40);
    cursor: pointer;
}

@media (min-width: 400px) {
    .window {
        width: calc((100% - 1*20px) / 1);
    }
}

@media (min-width: 550px) {
    .window {
        width: calc((100% - 1*20px) / 1);
    }
}

@media (min-width: 750px) {
    .window {
        width: calc((100% - 2*20px) / 2);
    }
}

@media (min-width: 1000px) {
    .window {
        width: calc((100% - 3*20px) / 3);
    }
}

@media (min-width: 1200px) {
    .window {
        width: calc((100% - 4*20px) / 4);
    }
}