Comment by yorwba

There are many ways to hide text. Low contrast, small font size, image covering part of the text, too-small box cutting off some parts, custom font making certain words look like other ones... Alerting the user about such formatting issues would be helpful (e.g. also when you try to redact something by drawing a black rectangle over it without removing the text underneath) but you probably shouldn't rely on it for security.

As long as Copilot can't be prevented from acting on instructions in its input, it would be safer to not make untrusted document content part of the input, similar to how macros in untrusted documents aren't executed by default.