In Gmail (and Outlook), each row of the message list shows:

| sender | subject | beginning of the body |

The preview is often useful, it tells you more about the message than the subject alone. I used Gmail a lot, and the preview really helps you scan the list and find emails faster.
In Proton webmail, the list has no preview.

I understand why Proton didn't did it like Gmail, because messages are only decrypted when you open them.
But with content search enabled, the content is already decrypted on the computer. The search index stores the plain-text body of each message, and the web app keeps a decrypted copy of it in memory.

So for these users, a one-line preview would just be a local lookup. The content is already there, so we might as well use it. (I made a working test by injecting JavaScript).

Suggestion
- An opt-in setting "Show message preview in the list", available when content search is enabled.
- Row layout : grey preview text after the subject (see Gmail for exemple). And for column layout/mobile, one line under it.
- No preview for messages not (yet) in the index.

Mockup exemple image : https://imgur.com/a/J6lEGza
(density is higher and text colors are different because I use some CSS tweaks.)

It can look messy at first glance, but once you have been used to that in other webmail, it is hard to comme back to having only the subject.