Hi Slavko

Try to create a new popup layout, but without scroll container. Use only div's and make header, main content and footer for your layout and use styling to fix it on top/bottom. I can send you an example of how you can do it.

CSS for that:

Just change the Variables based on your project.

//Popup
.popup-wrapper {
    display: flex;
    align-items: stretch;
    min-width: 768px;
    height:100%;
    position: relative;
    z-index: 3;
    background: $valcon-white;

    &.gray {
        background: $valcon-primary-lightgrey;
    }

    .main-content {
        width: 768px;
        display: flex;
        flex-direction: column;

        .col-header,
        .col-footer {
            flex-shrink: 0;
            padding: $spacing-medium $spacing-large;
            z-index: 1;
        }

        .col-header {
            position: relative;
            // min-height: 80px;
            box-shadow: 0px 4px 4px $shadow;

            h1 {
                padding-top: 0;
                padding-bottom: 0;
                padding-right: 0;
                line-height: 24px !important;
            }

            .close-page {
                position: absolute;
                right: 0;
                top: 0;
            }

            .header-icon {
                color: $valcon-primary-darkgrey; 
                text-indent: 100%;
                overflow: hidden;
                display: block !important;
                padding: 16px 24px;
                width: 48px;
                height: 32px;

                &:hover {
                    text-decoration: none;
                    color: $gray;
                }

                &:focus {
                    text-decoration: none;
                }
            }
            .mx-title {
                line-height: $font-size-h1 !important;
            }
            .header-tagline {
                display: block;
                color: $gray;
                font-size: $font-size-h5;
                margin-top: $spacing-smaller;
                max-width: 60%;
            }
        }

        .col-footer {
            height: 64px;
            box-shadow: 0px -4px 4px $shadow;
        }

        .col-content {
            flex-grow: 1;
            overflow: auto;
            padding: $spacing-large;
            // overflow-x: hidden; // CHECK THIS
        }
    }

    .additional-content {
        background: $valcon-primary-lightgrey;
        width: 0px;
        transition: width .3s ease-in-out;
        height: 100%;
        overflow-x: hidden;
        overflow-y: auto;

        & > * {
            width: 400px;
            display: block;
        }

        &.visible {
            width: 400px;
            transition: width .2s ease-in-out;

            .mx-listview li {
                padding: 0 16px;
            }
        }

        .close-menu {
            left: 0;
            top: 0;
            color: $valcon-primary-darkgrey; 
            background: $valcon-primary-lightgrey;
            display: block !important;
            padding: 16px;
            position: sticky; top: 0px;

            .back-icon {
                width:16px;
                height: 16px;
            }
            
            &:hover {
                text-decoration: none;
                color: $gray;
            }

            &:focus {
                text-decoration: none;
            }
        }

        .private-content {
            min-height: 250px;
        }

        .text-detail {
            color: $gray-dark !important;
        }
    }

    
}

.modal-dialog.mx-window.flexlayout_popup {
    width: auto !important;

    &.right-fluid {
        width: auto !important;
        top: 0 !important;
        right: 0 !important;
        left: unset !important;
        animation: slideinfromright 0.3s;

        .modal-body {
            height:100% !important;
        } 


        .col-content {    
            .content-768 {
                width:768px;

                height: 1300px;
            }
            .content-300 {
                width:300px;
                background: gray;
                height: 1300px;
                transition: width .3s ease-in-out;
            }
        }
    
    }
    
}
// Popup invisible close button
button.close-modal {
    top:0;
    left:0;
    opacity: 0;
    width: 100%;
    height: 100%;
    position: fixed;
    z-index:2;
}


// Built-in popups
.mx-underlay {
    background-color: $valcon-primary-darkgrey;
}

.modal-dialog {
    &.mx-window {
        width: 500px;
    }
    
    .modal-content {
        background: $valcon-primary-lightgrey;
        border: none;
        box-shadow: none;

        .modal-header {
            border-bottom: none;

            h4 {
                font-size: $font-size-h1;
                padding-top: 0;
                font-weight: 600;
            }
        }
    }
}

Best Regards

Marko