	Platforms	Pros	Cons
Atom	Cross	built-in markdown file preview
Emacs	Cross
Gedit	Cross	print preview	no code folding
Geany	Cross	print preview; Terminal
IntelliJ IDEA	Cross
Notepad++	Win	print line numbers, Right-click menu	R syntax highlight not work with most themes
Qt Creator	Cross	split a window (even same file)
RStudio	Cross	folding, best IDE for R coding	not support drag and drop a file but SendTo works
Sublime	Cross
vi	Cross
Visual Studio Code	Cross	Integrated Terminal, Markdown preview	Should not include minimap function

HTML editors: see here.

Atom

Markdown preview feature is OK but it cannot export to pdf or HTML.
pandoc conversion to html/PDF is not good; scripts part has no rectangle, tables have no separate lines.

Packages

To install a new package, go to Preferences -> Install -> type the package name and hit ENTER.

To remove a package, just delete the related folder.

rm -rf ~/.atom/packages/markdown-toc

Markdown Preview Enhanced. It allows to view in browser (html) or export to PDF. Use r instead of R for R language syntax highlight. Another nice feature is the scrollings of the markdown file and preview are together.
Markdown TOC. It messes up the # sign in script blocks :( So I switch to RStudio which has a good support to create the TOC (YAML header).