You can tell the modal which elements should no longer be scrollable when it's open. This is made possible by the stopScrollElements property, which takes a function with an optional ModalRemote parameter that can be used to determine which element should be blocked from scrolling.

The function given to stopScrollElements must return an array of HTML elements. By using the modal's anchor and tracing its parents, we can find the scrollable element to stop. It's also possible to bind the precise element you wish to stop scrolling or to use document.querySelector('something').

const stopScrollElements = (modal) => [modal.anchor?.parentElement?.parentElement] 

<button >
	Test
	<Modal {stopScrollElements} >
		Content
	</Modal>
</button>