[maho]

I really like https://sequencediagram.org/

- syntax is really close to PlantUML

- it's rendered in the browser -> immediate feedback

- great autocomplete and syntax-check in the code editor

- great connection between graphic and text (click on element -> jump to relevant line)

- graphical drawing and reordering

- presentation view (fullscreen + keeps actors "sticky" on top)

Especially dragging/dropping actors around is a lot of fun when you try to find the most suitable presentation format for complex diagrams. I'm not aware of any other editor that has this good integration between the code-editor and the visual editor.

It's not open source, but it's been around for free for a long time.