`hook global RegisterModified '/' %{ add-highlighter -override global/search regex "%reg{/}" 0:+u }` will underline the current search pattern

There is no ignorecase option, `(?i)` is the way to enable that in a regex, you can `map global normal / /(?i)` to auto insert-it

incsearch is the same

`add-highlighter global/ number-lines` display line numbers for all buffers

There is no swap file in Kakoune.