A major missing feature is the ability to search scrollback, i.e. cmd+f on Mac, ctrl+F on Linux.

This issue can be implemented in multiple steps, not just one giant PR:

• Core, single-thread search functionality in src/terminal (Naive search internals (core only) #2885, Considerably more search internals #9585)
• Search thread, mailbox on top of the core system utilizing the big renderer lock to read the screen for small time slices
• Hook up the search thread to Surface
• Rendering search matches in our renderers (non-GUI work)
• Search iteration (prev/next match) search: navigable search results (previous/next) #9702
• Glue in apprt, bindings macOS Search GUI #9709
• macOS GUI macOS Search GUI #9709
• GTK GUI GTK: Search UI #9756

EDIT: There was a previous approach here (see edit history) with concurrent PageList access. After various false starts, I think I've identified a better path forward.