v1.0.0

Install: download QuickLook.Plugin.QuickEdit.qlplugin, select it in Explorer, press Space, click Install, restart QuickLook.

Edit a few words of a Markdown file without leaving its QuickLook preview:

• Double-click rendered text (paragraph, heading, list item, table cell, quote or code block) and type. A double-clicked word is selected, so typing replaces it.
• Enter or clicking elsewhere finishes the edit; Esc cancels it.
• Ctrl+S writes the file. Nothing is saved otherwise: closing the preview discards unsaved changes.
• Ctrl+Z / Ctrl+Y undo and redo finished edits.

The preview is still QuickLook's own Markdown viewer (table of contents, highlighting, diagrams). Space, Esc and the arrows work as usual whenever you aren't typing.

Edits are written back as Markdown source: formatting around the change is kept (**subfolders** → **folders**), and an edit that can't be placed exactly is undone rather than guessed. Saving keeps the file's encoding and line endings, and is refused if the file changed on disk after the preview opened.

Tested on QuickLook 4.5.0.