IPython/html/static/notebook/less/widgetarea.less

div.widget_area {
    page-break-inside: avoid;
    .vbox();
}

div.widget_hbox {
    .hbox();
}

div.widget_vbox {
    .vbox();
}

div.widget_item {
    display: inline-block;
}

/* This class is for the widget subarea inside the widget_area and after
   the prompt div. */
div.widget_subarea {
    padding: 0.44em 0.4em 0.4em 1px;
    margin-left: 6px;
    .box-flex1();
}