Pau, this is awesome work.
It makes sense to put sorting, search and browse all in the same header area. Those features are all doing similar functions -- helping people find the conversations they're most interested in. But figuring out how to squeeze all three into the same space, plus advanced bonus options, is challenging.
Your solution for switching between the neutral/browsing/searching states on the Overview and Search slides looks really good to me. The user gets a call to action for both search and browse when they open the page, and the header switches focus between search and browse, depending on which one is more relevant to what the user's doing.
This design also downplays the sorting element in the header, which has to disappear from the header when the user scrolls down anyway. I don't know how valuable people find the sorting right now; this will help us to find out. :)
I do think that the advanced search and filter features get a little confusing by the end. There's a lot of power and customization in this design, and that brings a lot of signals to process.
For example, on the Browsing (ToC) slide, the topic titles on the left side of the panel use dark gray/light gray to indicate whether a topic is open or closed, but all of the icons are light gray on the right side of the panel -- except for the one that's blue, which indicates that there's recent activity on that topic. When you add in the faint-to-bright yellow highlighting in the next slide, that's a lot of different pieces of information marked by changes in color and contrast.
We may need to figure out the use cases for filtering and advanced search, and do a rough-draft priority ranking -- maybe starting with you, me, Nick, and whoever's interested, and then opening it up for the user research sessions?
And hooray for the user research -- we haven't done any research sessions on new features since I've been on the team, and I really want to. :) How has it worked on other teams?
Danny