Googleスプレッドシートの異常な挙動に長らく悩まされていたが、どうやらautocopy系のアドオンが悪さをしていると判明。
こちらのアドオンはGoogle Docsで動作しないようにされてる配慮がとても素晴らしいです。

nice copy

Sometimes doesn't work, but still...

Doesn't have 'Copy on select's right click copying issue. Saves me more time from having to deselect before using the right click menu from accidental copying what was selected previously.

It works for me perfectly and just what i needed thanks.

splendid.
Updated version of Copy On Select.