Add left border to additional header row in kableExtra

[karawoo]

When I add a header row to a table that has left and right outer borders, the border does not extend up through the additional header row. But, for some reason, a border is added on the right even though the rest of the table does not have a right border. Is there a way I can get a border on the left of an additional header row?

This seems like perhaps the opposite issue from issue #280.

---
title: "Untitled"
output: 
  pdf_document
header-includes:
  - \usepackage{makecell}
  - \usepackage{multirow}
---

```{r test, message = FALSE}
library("tidyverse")
library("kableExtra")

## Create data
dat <- tibble(
  x = 1:5,
  y = 6:10
)

dat %>%
  kable("latex", escape = FALSE) %>%
  column_spec(1, border_left = TRUE) %>%
  add_header_above(c("combo" = 2))
```

[Emm13B33]

I am also very interested in a solution for this when knitting to pdf. In a similar scenario, I have used column_spec() to add vlines on both the left and right edges of my table, but they are not extending into an additional header row I have created using add_header_above().

[jdb]

The right border is always added by add_header_above as seen in

haozhu233/kableExtra/blob/e69a890301f8c13047f4bc3453f944de7211be5f/R/add_header_above.R#L269-L271

I've made changes in my fork to allow for the creation of borders in headers (see below code), but there might be other ways of doing this.

---
title: "Untitled"
output:
  pdf_document: default
header-includes:
- \usepackage{makecell}
- \usepackage{multirow}
---

```{r test, message = FALSE}
library("tidyverse")
library("kableExtra")

## Create data
dat <- tibble(
  x = 1:5,
  y = 6:10
)

dat %>%
  kable(escape = FALSE) %>%
  column_spec(1, border_left = TRUE) %>%
  column_spec(2, border_right = TRUE) %>%
  add_header_above(c("combo" = 2), align = "|c|", escape = FALSE)
```

[karawoo]

I've opened an issue on GitHub for this.