I may be wrong, but it seems that the text input from the Vim window is what's parsed during the posting process to WordPress. The result is that, whether using textwidth or wrap/wrapmargin with linebreak, any linebreaks that appear in the text within Vim are placed in the final output (the WordPress post). Writing long posts or pages in Vim without proper line breaks/word wrapping can be hard on the eyes and can make for tedious editing. Is it possible to use visual wrapping and line breaks without the result appearing in the WordPress post?
Issue #7 new