converse.js/sass/_autocomplete.scss

#conversejs {
    [hidden] { display: none; }

    .visually-hidden {
        position: absolute;
        clip: rect(0, 0, 0, 0);
    }

    .form-group {
        .suggestion-box {
            width: 100%;
        }
    }

    .suggestion-box {
        position: relative;
        mark {
            background: var(--completion-light-color);
        }

        > input {
            display: block;
        }

        .suggestion-box__results,
        > ul {
            &:before {
                content: "";
                position: absolute;
                top: -.43em;
                left: 1em;
                width: 0; height: 0;
                padding: .4em;
                background: white;
                border: inherit;
                border-right: 0;
                border-bottom: 0;
                -webkit-transform: rotate(45deg);
                transform: rotate(45deg);
                z-index: -1;
            }
            position: absolute;
            left: 0;
            right: 0;
            z-index: 2;
            min-width: 100%;
            box-sizing: border-box;
            list-style: none;
            padding: 0;
            border-radius: .3em;
            margin: .2em 0 0;
            background: hsla(0,0%,100%,.9);
            background: linear-gradient(to bottom right, white, hsla(0,0%,100%,.9));
            border: 1px solid rgba(0,0,0,.3);
            box-shadow: .05em .2em .6em rgba(0,0,0,.1);
            text-shadow: none;

            > li {
                text-overflow: ellipsis;
                overflow-x: hidden;
                position: relative;
                cursor: pointer;
                padding: 1em;
            }
        }
        .suggestion-box__results--below {
            top: 2em;
        }
        .suggestion-box__results--above {
            bottom: 4.5em;
            &:before {
                display: none;
            }
            &:after {
                z-index: -1;
                content: "";
                position: absolute;
                bottom: -0.43em;
                left: 1em;
                width: 0;
                height: 0;
                padding: 0.4em;
                background: white;
                border: inherit;
                border-left: 0;
                border-top: 0;
                -webkit-transform: rotate(45deg);
                transform: rotate(45deg);
            }
        }
    }

    .suggestion-box > ul[hidden],
    .suggestion-box > ul:empty {
        display: none;
    }

    @supports (transform: scale(0)) {
        .suggestion-box > ul {
            transition: .3s cubic-bezier(.4,.2,.5,1.4);
            transform-origin: 1.43em -.43em;
        }
        
        .suggestion-box > ul[hidden],
        .suggestion-box > ul:empty {
            opacity: 0;
            transform: scale(0);
            display: block;
            transition-timing-function: ease;
        }
    }
    
    .suggestion-box > ul > li[aria-selected="true"] {
        background: var(--completion-dark-color);
        color: var(--inverse-link-color);
    }
    
    .suggestion-box li:hover mark {
        background: var(--completion-light-color);
        color: var(--inverse-link-color);
    }
    
    .suggestion-box li[aria-selected="true"] mark {
        background: var(--completion-normal-color);
        color: inherit;
    }
}

#conversejs.converse-fullscreen {
    .suggestion-box__results--above {
        bottom: 4.5em;
    }
}

#conversejs.converse-overlayed {
    .suggestion-box__results--above {
        bottom: 3.5em;
    }
}