This is the CSS I tried:

.menupopup-arrowscrollbox, .menupopup-arrowscrollbox::part(content), &::part(content) {
	box-shadow: none !important;
    border: none !important;
}

menupopup, panel {
    --panel-shadow: none !important;
}

menupopup > menu {
    box-shadow: none !important;
}