execCommand("copy") and execCommand("cut") should be allowed to modify the system clipboard in the context of a user gesture (the spec uses the language "allowed to show a popup").
Docs: https://developer.mozilla.org/en-US/docs/Web/API/document.execCommand http://updates.html5rocks.com/2015/04/cut-and-copy-commands
Samples: http://jsbin.com/yanamebope/quiet https://github.com/GoogleChrome/samples/tree/gh-pages/cut-and-copy