CupertinoContextMenu uses the ShaderMask widget, which currently isn't supported on web. So, when a CupertinoContextMenu is used on the web, an obscure error is thrown. The docs for CupertinoContextMenu has a broken dartpad example because of this problem.

Update: CupertinoContextMenu is usable on web now, but it is missing the small darkening/lightening effect on the target before the menu opens. Fixing that depends on #44152.