Attaching to existing tabs
In chromote: Headless Chrome Web Browser Interface

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

library(chromote)
r <- Chromote$new()

When you use ChromoteSession$new() or b$new_session(), you're typically connecting to an existing browser, but creating a new tab to attach to. It's also possible to attach to an existing browser and and existing tab. In Chrome debugging terminology a tab is called a "Target", and there is a command to retrieve the list of current Targets:

r$Target$getTargets()

Every target has a unique identifier string associated with it called the targetId; "9DAE349A3A533718ED9E17441BA5159B" is an example of one.

Here we define a function that retrieves the ID of the first Target (tab) from a Chromote object:

first_id <- function(r) {
  ts <- r$Target$getTargets()$targetInfos
  stopifnot(length(ts) > 0)
  r$Target$getTargets()$targetInfos[[1]]$targetId
}

The following code shows an alert box in the first tab, whatever it is:

rc <- ChromeRemote$new(host = "localhost", port = 9222)
r <- Chromote$new(browser = rc)
tid <- first_id(r)
b <- r$new_session(targetId = tid)
b$Runtime$evaluate('alert("this is the first tab")')