How to stop the background from moving top of the page when popup model opens in Magento 2

I created a Popup modal. when user click on button i showing the popup form. But the Background is moving top of the page when the popup modal is opened. How to stop the background from moving top when the popup is opened.

Thanks.

this is my popup modal form

<div id="popup-modal"> 
<div class="contact-form-popup" style="display:none;">

    <form class="form contact"
          action=""
          id="contact-form"
          method="post"
          data-hasrequired="<?= $block->escapeHtmlAttr(__('* Required Fields')) ?>"
          data-mage-init='{"validation":{}}'
          autocomplete="off">
        <fieldset class="fieldset">
            <legend class="legend"><span><?= $block->escapeHtml(__('Get in touch')) ?></span></legend>
            <div class="field note"><?= $block->escapeHtml(__('Simply fill in your details and enquiry below and one of our team will be in touch as soon as possible.')) ?></div>
            <div class="field firstname">
                <label class="label" for="name"><span><?= $block->escapeHtml(__('Your Name:')) ?></span></label>
                <div class="required-field">Required</div>
                <div class="control">
                    <input name="name" id="name" title="<?= $block->escapeHtmlAttr(__('Name')) ?>"
                           class="input-text" type="text" data-validate="{required:true}"/>
                </div>
            </div>
            
            <div class="field emailaddress">
                <label class="label" for="emailaddress"><span><?= $block->escapeHtml(__('Customer email address:')) ?></span></label>
                <div class="required-field">Required</div>
                <div class="control">
                    <input name="emailaddress" id="emailaddress" title="<?= $block->escapeHtmlAttr(__('Email')) ?>"
                           class="input-text" type="email" data-validate="{required:true, 'validate-email':true}"/>
                </div>
            </div>
            <div class="field mobile">
                <label class="label" for="mobile"><span><?= $block->escapeHtml(__('Phone number:')) ?></span></label>
                
                <div class="required-field">Required</div>
                <div class="control">
                    <input name="mobile" id="mobile" title="<?= $block->escapeHtmlAttr(__('Phone number')) ?>"
                           class="input-text" type="text" data-validate="{required:true}"/>
                </div>
            </div>
            <div class="field postcode">
                <label class="label" for="postcode"><span><?= $block->escapeHtml(__('Customer postcode:')) ?></span></label>
                <div class="required-field">Required</div>
                <div class="control">
                    <input name="postcode" id="postcode" title="<?= $block->escapeHtmlAttr(__('Customer postcode')) ?>"
                           class="input-text" type="text" data-validate="{required:true}"/>
                </div>
            </div>
            <div class="field enquiry">
                <label class="label" for="enquiry"><span><?= $block->escapeHtml(__('Enquiry:')) ?></span></label>
                <div class="control">
                    <textarea name="enquiry" id="enquiry" title="<?= $block->escapeHtmlAttr(__('Enquiry')) ?>"
                        class="input-text" type="text" ></textarea>
                </div>
            </div>
        </fieldset>
        <div class="actions-toolbar">
            <div class="primary">
                <button type="submit" id='customer-contact' title="<?= $block->escapeHtmlAttr(__('Submit')) ?>"
                        class="action submit primary">
                    <span><?= $block->escapeHtml(__('Submit')) ?></span>
                </button>
            </div>
        </div>
    </form>
</div>
<div style="display:none" class="static-block-message">
    <legend class="legend"><span><?= $block->escapeHtml(__('Get in touch')) ?></span></legend>
    <div class="field note">
        <?= $block->escapeHtml(__('Simply fill in your details and enquiry below and one of our team will be in touch as soon as possible.')) ?>
        <strong><?= $block->escapeHtml(__('Thank you. A member of our team will be in touch shortly.'))?></strong>
    </div>
    
</div>
</div>

<script>
require(
    [
        'jquery',
        'Magento_Ui/js/modal/modal'
    ],
    function($, modal) {
        jQuery(document).ready(function(){
            var options = {
                type: 'popup',
                responsive: true,
                innerScroll: true,
                modalClass: 'get-in-touch',
                buttons: []
            };
            var openModal = modal(options, $('#popup-modal'));
            
            jQuery('body').on('click', '.demo-banner .pagebuilder-button-primary', function(){
                alert("hello");
                jQuery('.contact-form-popup').show();
                jQuery('.static-block-message').hide();
                jQuery('#popup-modal').modal('openModal');
            });

            jQuery('body').on('click', '#contact-form .action.submit', function(e){
                   
                    e.preventDefault();
                    //e.stopImmediatePropagation();
                    if(!(jQuery('#contact-form').valid())){
                        return false;
                    }
                    else if((jQuery('#contact-form').valid())){
                        jQuery.ajax({
                            type: 'post',
                            url: '<?php echo $block->getUrl("popup/action/index") ?>',
                            data: jQuery('#contact-form').serialize(),
                            cache: false,
                            showLoader: 'true',
                            success: function(response) {
                                //alert('success');
                                jQuery('.contact-form-popup').hide();
                                jQuery('.static-block-message').show();
                                
                            }
                        });
                    }
                    return false;
                
            });
        });
    }
);
</script>