Bug in code_folding: show (in YAML) + class.source = 'fold-hide' (in a chunk) hides code for the rest of the document

[Pozdniakov]

I want to do code folding and choose certain chunks where code will be shown initially as it is described here.

However, using code_folding: show in YAML header and class.source = 'fold-hide' in a chunk leads to unexpected behavior: all next chunks of code are hidden too.

When I do code_folding: hide, I encounter the same problem: using class.source = 'fold-hide' hides code both from the selected chunk and all the chunks below.

reprex

---
title: "try code folding"
output:
  html_document:
    code_folding: show
---

 ```{r}
# This source must be shown
 ```

```{r class.source = 'fold-hide'}
# This source must be hidden
```

```{r}
# This source must be shown
```

This results in:

##session_info

>  xfun::session_info('rmarkdown')
R version 4.0.2 (2020-06-22)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.2, RStudio 1.1.463

Locale: ru_RU.UTF-8 / ru_RU.UTF-8 / ru_RU.UTF-8 / C / ru_RU.UTF-8 / ru_RU.UTF-8

Package version:
  base64enc_0.1.3 digest_0.6.25   evaluate_0.14   glue_1.4.2      graphics_4.0.2 
  grDevices_4.0.2 highr_0.8       htmltools_0.5.0 jsonlite_1.7.1  knitr_1.30.1   
  magrittr_1.5    markdown_1.1    methods_4.0.2   mime_0.9        rlang_0.4.7    
  rmarkdown_2.3.9 stats_4.0.2     stringi_1.5.3   stringr_1.4.0   tinytex_0.26   
  tools_4.0.2     utils_4.0.2     xfun_0.17       yaml_2.2.1     

Pandoc version: 1.19.2.1

R Markdown log

  |..........                                                            |  14%
  ordinary text without R code

  |....................                                                  |  29%
label: unnamed-chunk-1
  |..............................                                        |  43%
  ordinary text without R code

  |........................................                              |  57%
label: unnamed-chunk-2 (with options) 
List of 1
 $ class.source: chr "fold-hide"

  |..................................................                    |  71%
  ordinary text without R code

  |............................................................          |  86%
label: unnamed-chunk-3
  |......................................................................| 100%
  ordinary text without R code


/Applications/RStudio.app/Contents/MacOS/pandoc/pandoc +RTS -K512m -RTS try_folding.utf8.md --to html4 --from markdown+autolink_bare_uris+tex_math_single_backslash --output try_folding.html --email-obfuscation none --self-contained --standalone --section-divs --template /Library/Frameworks/R.framework/Versions/4.0/Resources/library/rmarkdown/rmd/h/default.html --no-highlight --variable highlightjs=1 --variable 'theme:bootstrap' --include-in-header /var/folders/mb/3y6vs_2x2m1dzch9wpmd6qk80000gn/T//Rtmp8oTREt/rmarkdown-str9090a0bb667.html --mathjax --variable 'mathjax-url:https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML' --variable code_folding=show --variable code_menu=1 


processing file: try_folding.Rmd
output file: try_folding.knit.md


Output created: try_folding.html

---

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---

[atusy]

@Pozdniakov
I'm the one who introduced the feature with the bug...
It will be fixed by #1906.
You may try it by installing rmarkdown by

remotes::install_github('rstudio/rmarkdown#1906')

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