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