How can my plugin display a populated new post window

I’ve installed your plugin and it doesn’t work.

Feeds counter are updated correctly, but no feed are shown (and so no ‘Press this’ buttons).

After that when I install it I see several notices: are you testing your plugin with wp debugging active?

The best way, for me, is redirect users to the standard wordpress post creation page. But how pre populated it?

After a quick look on your code, I’ll give you the bones of a solution that should be improved and tailored to your plugin.

Sure there are more elegant way, but first coming into my mind is:

1. create and save a post with selected feed content, and set status as auto-draft.
2. redirect users to edit page for the just created posts

Something like:

Javascript: in the file ‘js/myblogthis.js’

jQuery().ready(function($) {
  $('.entry-tools a').click(function(e) {
  e.preventDefault();
  var $entry = $(this).closest('.entry');
  var $titleObj = $entry.find('.entry-title');
  var _title = $titleObj.text();
  var _url = $titleObj.parent().attr('href');
  var _by = $entry.find('.author').text();
  var _date = $entry.find('.date').text();
  var _content= $entry.find('.entry-content').html();
  $.ajax({
      url: ajaxurl, type: 'POST', dataType: 'json',
      data: {
        title: _title, url: _url, by: _by, date: _date, content: _content,
        action: 'blog_this', nonce: myblogthisData.nonce
      }
    }).done(
      function( data ) {
        if ( data && data.url ) { window.location.href = data.url
        } else { alert( "Error!" ); }
      }
    );
  });
});

PHP

add_action('admin_enqueue_scripts', 'enqueue_blog_this_js');

add_action('wp_ajax_blog_this', 'my_blog_this');   

function enqueue_blog_this_js($page) {
  if ( $page != 'toplevel_page_orbital' ) return;
  wp_enqueue_script('myblogthis', plugins_url('js/myblogthis.js', __FILE__), array(), null );
  $data = array( 'nonce' => wp_create_nonce('myblogthis') );
  wp_localize_script('myblogthis', 'myblogthisData', $data);
}

function my_blog_this() {
  error_reporting(0);
  if ( ! isset($_POST['nonce']) || ! wp_verify_nonce($_POST['nonce'], 'myblogthis') )
    wp_send_json( array('error' => 'Security Fail') );
  $title = isset($_POST['title']) ? $_POST['title'] : '';
  $url = isset($_POST['url']) ? $_POST['url'] : '';
  $by = isset($_POST['by']) ? $_POST['by'] : '';
  $date = isset($_POST['date']) ? $_POST['date'] : '';
  $content = isset($_POST['content']) ? $_POST['content'] : '';
  if ( $title && $url && $by && $date) {
    $content .= sprintf(
      '<p>Originally posted by: %s on %s (see: <a href="https://wordpress.stackexchange.com/questions/112508/%s">%s</a>)</p>',
      esc_html($by), esc_html($date), esc_url($url), esc_html($title)
    );
  }
  $args = array(
    'post_status' => 'auto-draft',
    'post_title' => '',
    'post_author' => wp_get_current_user()->ID,
    'post_content' => $content
  );
  $id = wp_insert_post($args);
  if ( $id ) {
    $url = add_query_arg( array('action'=> 'edit', 'post'=> $id), admin_url('post.php') );
    wp_send_json( array('url'=> $url ) );
  }
  wp_send_json( array('error' => 'Saving Post Error') );
}

As you can see, I tried to create js according to the html outputted by ‘mainwindow.php’.

The main between the method you are using and the mine is that you let users select a text to re-blog, while I use the entire content.
But with some js improvements maybe you can modify this behaviour.