When I was implementing it in KMail, I was looking at Thunderbird, and envious of their fancy search UI. I had plans of implementing something similar, but it never panned out.
This is the so-called "quick search" by the way, which works as a filter and displays results in the inbox. There is also another search feature in Thunderbird which renders results differently. I never got used to that, and never used it, as I found its semantics unpredictable.
When I was implementing it in KMail, I was looking at Thunderbird, and envious of their fancy search UI. I had plans of implementing something similar, but it never panned out.