Minor progress bar infelicity

[hadley]

I am loving the new progress bar! But I noticed a couple of small issues exemplified by this simple Rmd:

```{r alpha}
Sys.sleep(0.5)

Sys.sleep(0.5)

Sys.sleep(5)

After alpha and bravo have completed and charlie is still running, I see this:

<img width="804" alt="Screenshot 2023-02-08 at 13 00 06" src="https://user-images.githubusercontent.com/4196/217626563-64eee875-9160-4ca9-a7cf-f834702e3e80.png">

Two small problems:

* Weirdly, the `processing` message appears part way in the middle of the progress bar
* The progress bar shows 100% complete even though we're still working on chunk 3.

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[hadley]

Even more nitpicky, I noticed that if I run rmarkdown::render() there are two blank lines before the progress bar starts:

> rmarkdown::render("delay.Rmd")

processing file: delay.Rmd
  |........................                                                 |  33% (alpha)  

[hadley]

And maybe there's one extra empty line at the bottom too? (at least when run_pandoc = FALSE).

> invisible(rmarkdown::render("delay.Rmd", run_pandoc = FALSE))

processing file: delay.Rmd

output file: delay.knit.md

> 

[hadley]

Hmmm, the first problem (where "processing..." is interleaved in the output") might actually be an RStudio problem

[hadley]

It is an RStudio bug: https://github.com/rstudio/rstudio/issues/8160. knitr could work around it by outputing the "Process file" message to stdout() instead of stderr(). We have some techniques to do this in rlang/cli, but you could also just switch the message() to cat().

@kevinushey pointed me to this article on why interleaving streams is generally hard: https://unix.stackexchange.com/questions/476080/what-prevents-stdout-stderr-from-interleaving/476089#476089. You might also argue that maybe the problem is that the progress bar is using stdout() since everything that appears in this display will be using stderr()

[hadley]

Not 100% sure this is the same problem, but note that in this connnect log the progress bar shows up after the error.

[kevinushey]

This is almost surely an RStudio issue -- not sure if there's a way to replicate the same flow in a "regular" R instance outside of RStudio.