kable's collapse_rows() and border

Please see the following kable:

df <- data.frame(
  C1 = 1:2, 
  C2 = c("a", "a"), 
  C3 = 3:4, 
  C4 = 5:6, 
  C5 = c("b", "b"), 
  row.names = c("Row 1", "Row 2")
  )
df %>% 
  kable() %>%
  add_header_above(c("", "Group 1" = 2, "", "Group 2" = 2), 
                   extra_css = "border-bottom: 2px solid green;") %>%
  kable_styling(full_width = FALSE) %>%
  collapse_rows(columns = c(3, 6)) %>% 
  row_spec(0, extra_css = "border-bottom: 2px solid;") %>%
  row_spec(1, extra_css = "border-bottom: 1px solid red;") %>%
  row_spec(2, extra_css = "border-bottom: 2px solid blue;")

It looks like this:

I have two problems:

• I would like to completely remove the line between Row 1 and Row 2.
• I would like to have the entire line below Row 2.

I added red and blue colors just to show what is happening.

After using collapse_rows(), row_spec() treats the bottom of columns C2 and C5 as belonging to row 1.

So when I edit row_spec(1, ...) (see red color), I end up editing lines in different rows.
Same with row_spec(2, ...) (see blue color), I cannot edit the lines of the columns with collapsed rows.

I tried to use column_spec(), but it is also not working well. For example,

column_spec(3, extra_css = "border-bottom: 2px solid brown;")

adds a line to columns C2 and C3,

column_spec(4, extra_css = "border-bottom: 2px solid brown;")

adds lines to different rows, and

column_spec(5, extra_css = "border-bottom: 2px solid brown;")

gives an error instead of adding a line under C5.

I was expecting to have this:

I tried to use border-bottom: none; to get rid of the red line part, but the line remains visible.

Any help appreciated, thanks in advance.

This should do the trick. Using a bit of trial and error, you can control the lines with extra row and column specs. It seems the column_specs only work if they are placed before the call to collapse rows.

PS I had to work with the development version of kableExtra: devtools::install_github("haozhu233/kableExtra") as there seems to be an issue with collapse_rows: https://github.com/haozhu233/kableExtra/issues/706

library(kableExtra)
  df |> 
  kable(fomat = "latex") |>
  add_header_above(c("", "Group 1" = 2, "", "Group 2" = 2), 
                   extra_css = "border-bottom: 2px solid green;") |>
  kable_styling(full_width = FALSE) |>
  column_spec(3, extra_css = "border-bottom: 2px solid;") |> 
  column_spec(6, extra_css = "border-bottom: 2px solid;") |> 
  collapse_rows(columns = c(3, 6)) |> 
  row_spec(0, extra_css = "border-bottom: 2px solid;") |> 
  row_spec(2, extra_css = "border-top: 1px solid white;") |>
  row_spec(2, extra_css = "border-bottom: 2px solid;")