LWC - modal does not display as a pop up

Question

I have a datatable with a column button "Open". Clicking this button should open a modal. The modal is shown under the component, not as a pop up. Could you point out my error please?

<template>
        <div style="height: 300px;">
            <lightning-datatable
                    key-field="id"
                    data={data.data}
                    columns={columns}
                    hide-checkbox-column
                    onrowaction={handleAction}>
            </lightning-datatable>
        </div>    
        <template if:true={openModal}>
                <div class="slds-modal__container">
                        <button class="slds-button slds-button_icon slds-modal__close slds-button_icon-inverse">
                          <svg class="slds-button__icon slds-button__icon_large" aria-hidden="true">
                            <use xlink:href="/assets/icons/utility-sprite/svg/symbols.svg#close"></use>
                          </svg>
                          <span class="slds-assistive-text">Cancel and close</span>
                        </button>
                        <div class="slds-modal__header">
                          <h1 id="modal-heading-01" class="slds-modal__title slds-hyphenate">Modal header</h1>
                        </div>
                        <div class="slds-modal__content slds-p-around_medium" id="modal-content-id-1">
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                        </div>
                        <div class="slds-modal__footer">
                          <button class="slds-button slds-button_neutral" aria-label="Cancel and close">Cancel</button>
                          <button class="slds-button slds-button_brand">Save</button>
                        </div>
                      </div>
        </template>
</template>

import { LightningElement, api, wire } from 'lwc';
import { NavigationMixin } from 'lightning/navigation';
import accounts from '@salesforce/apex/utils.getAccs';

const columns = [
    { label: 'Name', fieldName: 'Name' },
    { label: 'id', fieldName: 'Id'},
    { label: 'Action', type: 'button', typeAttributes: {
        iconName: 'action:preview',  
        label: 'View',  
        name: 'View',  
        title: 'View',  
        disabled: false,  
        value: 'view',  
        iconPosition: 'right'  
    }},
    { label: 'Action', type: 'button', typeAttributes: {
        iconName: 'action:preview',  
        label: 'Open Modal',  
        name: 'Open',  
        title: 'Open',  
        disabled: false,  
        value: 'open',  
        iconPosition: 'right'  
    }}
];

    export default class DataTable extends NavigationMixin(LightningElement) {
        @api recordId;
        @wire(accounts) data;
    
        openModal = false;
    
        columns = columns;
    
        handleAction(event){
            if(event.detail.action.name=="View"){
                console.log(event.detail.action.name); //View
                console.log(event.detail.row.Name); //Name
                console.log(event.detail.row.Id); //Id
    
                this[NavigationMixin.Navigate]({
                    type: 'standard__recordPage',
                    attributes: {
                        recordId: event.detail.row.Id,
                        actionName: 'view'
                    },
                });
            }
            if(event.detail.action.name=="Open"){
                this.openModal = true;
            }
        }
    }

Answer 1

You are missing the outer slds-modal class (and also the backdrop) and that is likely to be the problem:

<section class="slds-modal slds-fade-in-open">
    ...
</section>
<div class="slds-backdrop slds-backdrop_open"></div>

See the SLDS Modals documentation.

Also this is the approach I have used to show/hide modals via CSS:

@api show = false;

get dialogClass() {
    const size = 'slds-modal_medium';
    const open = this.show ? 'slds-fade-in-open' : '';
    return `slds-modal ${size} ${open}`;
}

get backdropClass() {
    const open = this.show ? 'slds-backdrop_open' : '';
    return `slds-backdrop ${open}`;
}

with the the HTML then being:

<section class={dialogClass}>
    ...
</section>
<div class={backdropClass}></div>