Using md() in column label breaks column width in PDF output

[mpjashby]

Prework

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Description

In gt 1.3.0 (but not in 1.2.0, as far as I can tell), using md() inside a column label in a PDF table wrongly causes that column to be set to be the full width of the latex \linewidth.

Reproducible example

---
title: "Untitled"
format: pdf
---

This file illustrates a problem with using `md()` inside a PDF table column heading in gt 1.3.0 that causes the rest of the table to be placed off the page. 

This code works as expected and produces @tbl-works:

```{r}
#| label: tbl-works
#| tbl-cap: "Works as expected"

library(gt)

countrypops |>
  head(10) |>
  gt() |>
  cols_label(country_code_2 ~ "country code")
```

But if we use `md()` to break a long column name over multiple lines, the column is assigned the width of `\linewidth` so the rest of the table disappears off the right margin (@tbl-doesnt):

```{r}
#| label: tbl-doesnt
#| tbl-cap: "Does not work as expected"

countrypops |>
  head(10) |>
  gt() |>
  cols_label(country_code_2 ~ md("country<br>code"))
```

PDF output: gt_bug.pdf

Expected result

The column should keep the automatically-assigned width it had previously.

Session info

R version 4.5.2 (2025-10-31)
Platform: aarch64-apple-darwin20
Running under: macOS Tahoe 26.2

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib 
LAPACK: /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.12.1

locale:
[1] C.UTF-8/C.UTF-8/C.UTF-8/C/C.UTF-8/C.UTF-8

time zone: Europe/London
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] gt_1.3.0

loaded via a namespace (and not attached):
 [1] digest_0.6.39    utf8_1.2.6       R6_2.6.1         fastmap_1.2.0    tidyselect_1.2.1
 [6] magrittr_2.0.4   glue_1.8.0       tibble_3.3.1     pkgconfig_2.0.3  htmltools_0.5.9 
[11] dplyr_1.1.4      generics_0.1.4   lifecycle_1.0.5  xml2_1.5.2       cli_3.6.5       
[16] sass_0.4.10      vctrs_0.7.1      withr_3.0.2      compiler_4.5.2   tools_4.5.2     
[21] pillar_1.11.1    rlang_1.1.7      fs_1.6.6        

[bzkrouse]

@mpjashby I believe a workaround is to use cols_width() to set the column widths, then they are respected.