One-liner to show PSI viewer dialog. Normally it's only enabled in plugin projects.(This is a mini-plugin for https://github.com/dkandalov/live-plugin)

plugin.groovy

import com.intellij.internal.psiView.PsiViewerDialog
import static liveplugin.PluginUtil.*

registerAction("showPsiViewerDialog", "ctrl shift P") { event ->
  new PsiViewerDialog(project, currentEditorIn(event.project)).show()
}

The constructor has changed at some point, it is currently PsiViewerDialog(@NotNull Project project, @Nullable Editor selectedEditor) {

New version:

import com.intellij.internal.psiView.PsiViewerDialog
import static liveplugin.PluginUtil.*

registerAction("showPsiViewerDialog", "ctrl shift P") {
  new PsiViewerDialog(project, false, null, null).show()
}

For the current file, it's

import com.intellij.openapi.actionSystem.AnActionEvent
import com.intellij.internal.psiView.PsiViewerDialog

import static liveplugin.PluginUtil.*

registerAction("Show Psi Viewer Dialog for Current File", "ctrl shift P") { AnActionEvent e->
  // IntelliJ way
  //def editor = com.intellij.openapi.actionSystem.CommonDataKeys.EDITOR.getData(e.getDataContext())
  // live-plugin way
  new PsiViewerDialog(project, currentEditorIn(e.project)).show()
}