Select a chat in the list, tap in the menu and select a color to highlight that chat.
This could be an alternative solution to “mark as unread” and also a way to combine the chats for argument (example: yellow to read later, green for work, orange for friends…)
I suggest to use the classic three highlighter colors: green, yellow, orange.
We already have pinned chats, also “mark as unread” is resolved automatically when opening the chat again, but marking chats for work, friends, family is “permanent” marking right?
also there is still the challenge of multi device sync and maybe also a question about imap sync?
And my idea is a local setting, without sync.
Probably on your mobile you want to highlight different chats than on your desktop.