matrix-bot/themes/base/css/pages/MatrixChat.css

.mx_MatrixChat {
    position: relative;
    width: 100%;
    height: 100%;
}

.mx_MatrixChat_leftPanel {
    position: absolute;
    left: 0px;
    width: 250px;
    height: 100%;
}

.mx_MatrixChat_leftPanel .mx_MatrixToolbar {
    position: absolute;
    height: 20px;
}

.mx_MatrixChat_leftPanel .mx_RoomList {
    position: absolute;
    top: 20px;
    bottom: 0px;
    width: 250px;
    overflow-y: scroll;
}

.mx_MatrixChat .mx_RoomView {
    position: absolute;
    left: 255px;
    right: 0px;
    height: 100%;
}