@mattstibbs According to Can I Use it’s supported in recent versions of Firefox, https://caniuse.com/#feat=clipboard but that doesn’t appear to be true. There do seem to be ways of enabling it if you’re installing an extension, but nothing I could find for web apps.
Looks like document.execCommand(‘copy’) is the way to go for now. A little digging into the Anvil IDE shows that what they select is a hidden field, so you don’t actually see the selection on the visible field.