HTMLElement: beforetoggle イベント

addEventListener("beforetoggle", (event) => {});
onbeforetoggle = (event) => {};

const popover = document.getElementById("mypopover");
// ...
popover.addEventListener("beforetoggle", (event) => {
  if (event.newState === "open") {
    console.log("Popover is being shown");
  } else {
    console.log("Popover is being hidden");
  }
});

popover.addEventListener("beforetoggle", () => {
  //...
});
popover.showPopover();
popover.hidePopover();
// `beforetoggle` は 1 回しか発行されない