podlite

Podlite editor is a desktop app for writing structured documents with a live
preview: the source on the left, the rendered page on the right, updating as you
type.

Markdown files open here too. Podlite takes Markdown in, so a .md file is
highlighted as Markdown and rendered as Markdown — the same editor, not a
separate mode.

Podlite itself is a lightweight block-based markup language, with first-class
blocks for headings, lists, code, tables, diagrams, math and includes. It is not
tied to any domain, programming language or toolchain.

In the editor: Mermaid diagrams and LaTeX formulas render inline, code blocks are
syntax-highlighted by language, long sections fold, images can be pasted or
dropped in, and =include references resolve at render time.

Export to HTML and PDF. Free and open source under the MIT License.