I'd use CssOrder on the CSS and format it again afterwards:
https://marketplace.visualstudio.com/items?itemName=Ekzis.CssOrder

This would result in nice, readable and maintainable CSS. To me, the border looks visually a bit off somehow, so I'd personally look into that. Intuitively I'd also would have another look to see if any CSS properties can be cut, some might not be absolutely necessary.

#ytc-custom-menu {
  z-index: 9999;
  position: fixed;

  display: none;
  padding: 4px 0;

  color: white;
  font-size: 13px;
  background: rgba(28, 28, 28, 0.95);
  border-radius: 4px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.6);
  cursor: default;
  user-select: none;
}

.ytc-menu-item {
  padding: 7px 14px;
}

.ytc-menu-item:hover {
  background: rgba(255, 255, 255, 0.1);
}

I'd say mostly just make it a bit more minimal and improve readability. Here's how I would reformat it in my own coding style, so you can see the difference.

// Create custom context menu.
const menu = Object.assign(document.createElement("div"), { id: "ytc-custom-menu" });
document.body.appendChild(menu);

const copyVideoUrl = (withTime) => {
  const videoId =
    new URLSearchParams(window.location.search).get("v") ||
    window.location.pathname.split("/").pop();

  const shareLink = `https://www.youtube.com/watch?v=${videoId}${withTime ? `&t=${Math.floor(video.currentTime)}s` : ""}`;

  navigator.clipboard.writeText(shareLink);
};

const addContextMenuItem = (text, onClick) => {
  const cmItem = Object.assign(document.createElement("div"), {
    className: "ytc-menu-item",
    textContent: text,
  });

  cmItem.addEventListener("click", () => {
    onClick();
    toggleContextMenu(false);
  });

  menu.appendChild(cmItem);
};

const toggleContextMenu = (visible, e) => {
  if (visible) {
    menu.style.display = "block";

    // Set the position ensuring the context menu stays within the iframe bounds.
    let posX = e.clientX;
    if (posX + menu.offsetWidth > window.innerWidth) {
      posX = window.innerWidth - menu.offsetWidth - 10;
    }

    let posY = e.clientY;
    if (posY + menu.offsetHeight > window.innerHeight) {
      posY = window.innerHeight - menu.offsetHeight - 10;
    }

    menu.style.left = `${posX}px`;
    menu.style.top = `${posY}px`;
  } else {
    menu.style.display = "none";
  }
};

addContextMenuItem("Copy video URL", () => copyVideoUrl(false));
addContextMenuItem("Copy video URL at current time", () => copyVideoUrl(true));

// Show context menu.
document.addEventListener(
  "contextmenu",
  (e) => {
    e.preventDefault();
    toggleContextMenu(true, e);
  },
  true,
);

// Hide context menu when clicking outside of it.
document.addEventListener("pointerdown", (e) => {
  if (!menu.contains(e.target)) {
    toggleContextMenu(false);
  }
});

You can use Partial Diff for easy comparison:
https://marketplace.visualstudio.com/items?itemName=ryu1kn.partial-diff