One could imagine a design where even the app vendor is untrusted... You would send an encrypted chunk direct to the GPU, which would then decrypt and render the message text in some secure environment onto the screen.

Neither the OS nor the application would know the contents of your message beyond "it's 500x700 pixels".

Similar things are done for DRM video, and widevine level 1 or 2 haven't seen many breaches despite running on a wide array of hardware open to physical attack.