You can drag or modify the items in the diagram and it changes the UML, that's actually pretty neat. PlantUML normally only goes code to diagram.

It's pretty much that. I like to use https://www.websequencediagrams.com/ and/or plantuml plugins for vscode that can process locally or via api.

Or Mermaid, which is less capable but natively supported by GitLab and GitHub.

I really wish Mermaid and PlantUML had compatible syntax, or at least more overlap for the basic structural stuff. On a day to day basis I'm writing either PlantUML in Confluence (via a plugin) or Mermaid in GitHub, and I almost always have to open up the reference or review an existing diagram to refresh myself on the different syntax.

FYI there’s also a Mermaid plugin for Confluence, we use it extensively. If you don’t mind losing some features from PlantUML for the sake of consistency.

plant needs to have a UI to manipulate the drawings and round-trip the adjustments back into the spec. This is required for any non-trivial diagram.