This is (mostly) an internal re-architecture to use Periscope, a framework for Visual Studio debugging visualizers. There are two primary benefits:

• (Optional) Latest-version checking on load. If there's a later release, the visualizer will notify about it.
• The same UI components of the visualizer are now also available as a separate NuGet package, for both Antlr.Runtime and Antlr.Runtime.Standard. They can be embedded in third-party applications.

If you're debugging the current Antlr.Runtime.Standard, use the ParseTreeVisualizer.Standard.* appropriate for your version of Visual Studio. If you're using the older Antlr.Runtime, use the ParseTreeVisualizer.Legacy.*.