Looking for romance where the two protagonists interact normally for a while, may be friends, co-workers, anything. Maybe one person develops feelings which may or may not be unrequited and deliberates for a while about whether or not to confess. That situation is a good setup but not my main concern.

The important point is that when one person confesses, the other one immediately agrees, no misunderstandings, no contemplation. As if the person was waiting to get confessed to the whole time.

Maybe A-ko was unaware of B-san's feelings. Or, B-san could just be really easygoing and see no downside to starting a relationship. I prefer comedy over drama.

You could try Lovely Complex. It's one of my favorite manga and has most of what you're looking for.

Also, If you're into Yuri/Shoujo-ai, there are a bunch with similar scenario's.

Love Berrish! towards the end.

Lovely complex is almost like what I wanted but the guy was in shock for half a volume after the confession if I remember right.

I don't mind yuri. As long as it's not some surreal girls school. I find things like Maria-sama ga Miteru ...discombobulating.
Sasameki Koto may or may not turn into what I want but I highly doubt it because anytime one character thinks about confessing drama manifests and multiplies.

What I'm looking for is similar to Kimi ni Todoke, if Sawako had confessed, and then Kazehaya turned around and said "Alright!" and did a guts pose. Instead of... whatever did happen.