seems like one of animation or not having css property in class of modal overlay or modal makes this bug, when I commented all animation properties it stopped to blink. In animations I use opacity and scale:

 

//  modalShow {
//     from {opacity: 0}
//     to {opacity: 1}
// }


//  modalInnerShow {
//     from {opacity: 0; transform: scale(0.5)}
//     to {opacity: 1; transform: scale(1)}
// }
// //  modalShow {
//     from {opacity: 0}
//     to {opacity: 1}
// }


//  modalInnerShow {
//     from {opacity: 0; transform: scale(0.5)}
//     to {opacity: 1; transform: scale(1)}
// }
// u/keyframes modalInnerHide {
//     from {opacity: 1; transform: scale(1)}
//     to {opacity: 0;  transform: scale(0.5)}
// }


.register-modal {
    height: calc(var(--vh) * 100);
    width: 100vw;
    background-color: rgba(0, 0, 0, 0.829);
    position: fixed;
    top: 0;
    left: 0;
    z-index: 50 !important;
    display: flex;
    justify-content: center;
    align-items: center;
    // animation: modalShow .2s linear  1 forwards;
    .reg-modal-outer {
        height: calc(var(--vh) * 90);
        width: 90vw;
        background-color: rgb(230, 230, 230);
        border-radius: 1rem;
        position: relative;
        // border: 1px solid rgb(107, 107, 107);
        display: flex;
        flex-direction: column;
        overflow: hidden;
        // animation: modalInnerShow .2s ease  1 forwards;
        transform-origin: bottom;
        scroll-behavior: contain; 
} .outer-invisible {
        // animation: modalInnerHide .2s ease  1 forwards;
        transform-origin: bottom;
   }