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?
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.
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.
If you do want to render, you don't have to do the convoluted PDF route. That's what the user would do, the software would just use its normal drawing code. OCR is neither needed nor desirable. OCR errors would erroneously flag perfectly normal text, and it can be fooled just like a human can. You can detect whether or not there's any rendering with the bounding box of whatever text section you are looking for (something Word already has to calculate if it wants to render correctly on screen).
I'm only talking about checking for visible text. This will not solve the larger problem of malicious AI instructions.