Since this relies on a local index, does that not mean that it needs to download all your email to be able to search it? And if I want to search on a new device it needs to download all my emails for me to search?
I activated the feature today, and my Indexed DB for mail.protonmail.com now has a table "messages", and each message has an ID, a time, and an "aesGcmCiphertext" object with 2 child objects of keys "iv" and "ciphertext". The "iv" object has 12 integer keys from 0-11, with integer values between 0-255. I don't see any data in the "ciphertext" object at all.
I am not well versed in indexeddb so can't shed more light.
I do not know how much of my inbox is attachments and embedded images though.
This means that when you have search message content enabled, but your web browser is closed, an attacker with physical access to your device cannot read the contents of the index file. If you are logged in with your ProtonMail tab open, a decrypted version is stored in memory, but at this point, they would have direct access to your emails, anyway.