Flickering when stepping through fallback vdocs

From @DavisVaughan  https://github.com/posit-dev/ark/pull/1046#discussion_r2849416380


```r
debugonce(mutate.data.frame)
mutate(mtcars, x = 1 + "x")
```

https://github.com/user-attachments/assets/38eb9104-87ad-4d4e-9c9b-6c1776555d9e