add a marker for when channels/spaces with unread messages are scrolled off the screen

[ask-compu]

Describe the problem

can be hard to find a space/channel with unread messages when it's scrolled off screen

Describe the solution you'd like

add some sort of indicator to the top/bottom of the left sidebar (for spaces with unread messages) or channel list (for channels with unread messages) that shows if any of the spaces/channels off screen in that scroll direction have unread messages

Alternatives considered

No response

Additional context

No response