.o_form_view {
    .o_calendar_warning {
        cursor: help;
    }

    .o_calendar_warning_tooltip {
        display: block;
        height: 0;
        opacity: 0;
        pointer-events: none;
    }

    .o_calendar_warning:hover + .o_calendar_warning_tooltip {
        display: inline;
        height: auto;
        opacity: 1;
        transition: opacity 0.5s linear;
    }
}