Получение селектора над вкладками для изменения размера и перемещения по мере необходимости?

Вместо того, чтобы использовать div «селектор», не могли бы вы просто применить стиль к активному тегу «a»?

Редактировать

Хорошо, я вижу, что вы пытаетесь сделать в отношении анимации селектора для выбранного элемента.

Добавьте «справа» и «снизу» в CSS .selector следующим образом:

.tabs .selector {
    height: 100%;
    display: inline-block;
    position: absolute;
    left: 0px;
    top: 0px;
    right:0px;
    bottom:0px;
    z-index: 1;
    border-radius: 50px;
    transition-duration: 0.6s;
    transition-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55);
    background: #05abe0;
    background: -moz-linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
    background: -webkit-linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
    background: linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#05abe0', endColorstr='#8200f4', GradientType=1);
}

И добавьте «верхнюю» позицию в следующую часть кода:

 $(".tabs").on("click", "a", function (e) {
        e.preventDefault();
        $('.tabs a').removeClass("active");
        $(this).addClass('active');
        var activeWidth = $(this).innerWidth();
        var itemPos = $(this).position();
        $(".selector").css({
            "left": itemPos.left + "px",
             "top": itemPos.top + "px",
            "width": activeWidth + "px"
        });

Редактировать 2

Чтобы установить все элементы одинаковой ширины, вы можете использовать:

  var maxWidth = 0;
var $element;
$(".tabs a").each(function(){
   $element = $(this);
   if ($element.width() > maxWidth){
     maxWidth = $element.width();
   }

});

  $(".tabs a").each(function(){
   $(this).width(maxWidth);

});

Редактировать 3

Это пример кода, который обсуждался в комментариях:

<script type = "text/javascript" src = "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<div class = "wrapper">
    <nav class = "tabs">
        <div class = "selector"></div>
        <a href = "javascript:void(0)" class = "active" data-id = "1"><i class = "fas fa-burn"></i>Avengers</a>
        <a href = "javascript:void(0)" data-id = "2"><i class = "fas fa-bomb"></i>Guardians of The Galaxy</a>
        <a href = "javascript:void(0)" data-id = "3"><i class = "fas fa-bolt"></i>Thor</a>
        <a href = "javascript:void(0)" data-id = "4"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "5"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "6"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "7"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "8"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "9"><i class = "fab fa-superpowers"></i>Black Panther</a>
        <a href = "javascript:void(0)" data-id = "10"><i class = "fab fa-superpowers"></i>Black Panther</a>

    </nav>
</div>

<div class = "tabs-content active" id = "content1">
    <p>
        Bacon ipsum dolor sit amet beef venison beef ribs kielbasa. Sausage pig leberkas, t-bone sirloin shoulder
        bresaola. Frankfurter rump porchetta ham. Pork belly prosciutto brisket meatloaf short ribs.
    </p>
    <p>
        Brisket meatball turkey short loin boudin leberkas meatloaf chuck andouille pork loin pastrami spare ribs
        pancetta rump. Frankfurter corned beef beef tenderloin short loin meatloaf swine ground round venison.
    </p>
</div>

<div class = "tabs-content" id = "content2">
    <p>
        Bacon ipsum dolor sit amet landjaeger sausage brisket, jerky drumstick fatback boudin ball tip turducken.
        Pork
        belly meatball t-bone bresaola tail filet mignon kevin turkey ribeye shank flank doner cow kielbasa shankle.
        Pig
        swine chicken hamburger, tenderloin turkey rump ball tip sirloin frankfurter meatloaf boudin brisket ham
        hock.
        Hamburger venison brisket tri-tip andouille pork belly ball tip short ribs biltong meatball chuck. Pork chop
        ribeye tail short ribs, beef hamburger meatball kielbasa rump corned beef porchetta landjaeger flank. Doner
        rump
        frankfurter meatball meatloaf, cow kevin pork pork loin venison fatback spare ribs salami beef ribs.
    </p>
    <p>
        Jerky jowl pork chop tongue, kielbasa shank venison. Capicola shank pig ribeye leberkas filet mignon brisket
        beef kevin tenderloin porchetta. Capicola fatback venison shank kielbasa, drumstick ribeye landjaeger beef
        kevin
        tail meatball pastrami prosciutto pancetta. Tail kevin spare ribs ground round ham ham hock brisket
        shoulder.
        Corned beef tri-tip leberkas flank sausage ham hock filet mignon beef ribs pancetta turkey.
    </p>
</div>


<script>

var maxWidth = 0;
var $element;
$(".tabs a").each(function(){
   $element = $(this);
   if ($element.width() > maxWidth){
     maxWidth = $element.width();
   }
});

$(window).on('resize', function(){
            $(".selector").hide();
      var activeItem = $(".tabs a[class*='active']");
      var id = activeItem.data("id");
      setSelectorPosition(id);
});

$(".tabs a").each(function(){
   $(this).width(maxWidth);

});

  // Set first active link
  setSelectorPosition(1);



    $(".tabs").on("click", "a", function (e) {
        e.preventDefault();
        $('.tabs a').removeClass("active");
        $(this).addClass('active');

        // Get id of link clicked
        var id = $(this).data("id");

        // Set selector position
        setSelectorPosition(id);

        // Hide all tabs
        $('.tabs-content').hide();

        // Show current tab
        $('#content' + id).show();
    });

    function setSelectorPosition(id) {          
            var activeItem = $(".tabs").find("[data-id='" + id + "']"); 
        var activeWidth = activeItem.innerWidth();
        var itemPos = activeItem.position();
        $(".selector").css({
            "left": itemPos.left + "px",
             "top": itemPos.top + "px",
            "width": activeItem.innerWidth() + "px",
            "height": activeItem.innerHeight() + "px"
        });
        $(".selector").show();
    }

</script>

<style>

@import url('https://fonts.googleapis.com/css?family=Roboto');
    @import url('https://use.fontawesome.com/releases/v5.0.13/css/all.css');

    body {
        font-family: 'Roboto', sans-serif;
    }

    .wrapper {

        text-align: center;

        margin: 50px auto;
    }

    .tabs {
        margin-top: 50px;
        font-size: 15px;
        list-style: none;
        background: #fff;
        box-shadow: 0px 5px 20px rgba(0, 0, 0, 0.1); 
        display: inline-block;

        border-radius: 50px;
        position: relative;
    }

    .tabs a {
        text-decoration: none;
        color: #777;
        text-transform: uppercase;
        padding: 10px 20px;
        display: inline-block;
        position: relative;
        z-index: 1;
        transition-duration: 0.6s;
    }

    .tabs a.active {
        color: #fff;
    }

    .tabs a i {
        margin-right: 5px;
    }

    .tabs .selector {
        display:none;
        height: 100%;
        position: absolute;
        left: 0px;
        top: 0px;
        right:0px;
        bottom:0px;
        z-index: 1;
        border-radius: 50px;
        transition-duration: 0.6s;
        transition-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55);
        background: #05abe0;
        background: -moz-linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
        background: -webkit-linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
        background: linear-gradient(45deg, #05abe0 0%, #8200f4 100%);
        filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#05abe0', endColorstr='#8200f4', GradientType=1);
    }


    .tabs-content {
        display: none;
    }

    .tabs-content.active {
        display: block;
    }

</style>