我不太清楚此时显示弹出对话框"是什么意思，但请执行以下操作:

I'm not exactly sure what do you mean under "show popup dialog at this point", but do something like this:

IEditorPart editor =  PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage().getActiveEditor();
if (editor instanceof ITextEditor) {
  ISelectionProvider selectionProvider = ((ITextEditor)editor).getSelectionProvider();
  ISelection selection = selectionProvider.getSelection();
  if (selection instanceof ITextSelection) {
    ITextSelection textSelection = (ITextSelection)selection;
    int offset = textSelection.getOffset(); // etc.
  }
}

当然，在生产代码中做空检查等.

Of course, in production code do null checks etc.