On the masking of undisplayable characters

[bruce.connor.am]

I just noticed we have a feature where we use display tables to mask characters that cannot be displayed.

When this happens, it caused me quite a bit of confusion, as I tried to type one thing and something else got inserted. I eventually thought of investigating the character (which revealed it was being masked by the display table), but many users wouldn't know to do this.

Could we have a message to let the user know when this happens? Something like "undisplayable char being displayed as X".

I know this is meant to be helpful, but displaying something different from what's in the file is a pretty big deal. I'm not entirely sure it should be done without consent, but it definitely shouldn't be done silently.

Artur