Tab capture

You need to trap the focus inside the modal. For example, you can create an event listener that listens for the Tab key and moves focus up to the top element when it leaves the last element. If you want to avoid writing a focus trap yourself, you can use this one:

        // Run trapFocus on a modal that you want to trap focus within.
        // Shamelessly stolen from https://hidde.blog/using-javascript-to-trap-focus-in-an-element/
        function trapFocus(element) {
            var focusableEls = element.querySelectorAll('a[href]:not([disabled]), button:not([disabled]), textarea:not([disabled]), input[type="text"]:not([disabled]), input[type="radio"]:not([disabled]), input[type="checkbox"]:not([disabled]), select:not([disabled])');
            var firstFocusableEl = focusableEls[0];
            var lastFocusableEl = focusableEls[focusableEls.length - 1];
            var KEYCODE_TAB = 9;

            element.addEventListener('keydown', function (e) {
                var isTabPressed = (e.key === 'Tab' || e.keyCode === KEYCODE_TAB);

                if (!isTabPressed) {
                    return;
                }

                if (e.shiftKey) /* shift + tab */ {
                    if (document.activeElement === firstFocusableEl) {
                        lastFocusableEl.focus();
                        e.preventDefault();
                    }
                } else /* tab */ {
                    if (document.activeElement === lastFocusableEl) {
                        firstFocusableEl.focus();
                        e.preventDefault();
                    }
                }
            });
        }