Hello @cderv and thanks for answering.
I can live with bookdown::preview_chapter() or rmarkdown::render().
However it is not exactly what I am expecting. I may not have been clear enough so let me rephrase:
I have a classical book file tree, composed of _bookdown.yml, _output.yml, index.Rmd, and a few chapters.Rmd.
What I would like to do is to mimic the knit button behaviour, I mean rendering the chapters.Rmd in the Viewer pane (not in web browser) with a dedicated .yml different from the one used in index.Rmd.
Please note that I tried rmarkdown::render(input="Chapter1.Rmd", output_yaml="header.yml" ), where header.ymlis similar to a classical frontmatter yaml. It returned an error mentioning that arguments passed as title was not found. So I guess the header.yml is not understood as a yaml file, but I don't know why.
My motivation to do this is that I would like to be able to display the rendered content of my Chapters.Rmd in the Viewer pane and being able to run the chunk of codes without having to continuoulsy switch from the html file to the RStudio console and vice versa.
Thanks for your help!
Pierre