I may be naive here but can the hidden text not be flagged or outright removed before being passed to copilot? Why would there not be consideration for what a human user can see, especially if the hidden text was added by copilot in the first place?
Hide your prompt injection in terms & conditions, plain sight but totally invisible.
> Why would there not be consideration for what a human user can see,
How would a machine actually know which part of a document a human can see unless they print it to PDF, scan the rasterised PDF and compare the result from the OCR with text in the document?
I mean, I dunno how Word would decide that the following can't be seen by a user: white-on-white text, rendering off-page, embedded font with no lines, text covered by an image, etc.
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.