title: JavaFX - disappearing icons on toolbar buttons
body: <p>I'm writing JavaFX  application on JDK 8. The application has a menu and toolbar with some buttons. The problem is, when ever menu opens and covers toolbar buttons, icons on tollbar buttons disappear. Button functionality  remains unchanged, tooltips appear correctly, only icons are missing. The image bellow shows this:</p>
  
  <p><img src="https://i.stack.imgur.com/7VKDY.png" alt="enter image description here"></p>
  
  <p>I create toolbar buttons off the UI thread, but add them to the toolbar on UI thread.</p>
  
  <pre><code> // this is off UI thread
   newFileButton = new Button();
   newFileButton.setId(JFXFileMenuConstants.NEW_FILE_TOOLBAR_BUTTON);
   newFileButton.setGraphic((Node) handler.getResource("jfx.gui.file.new.menuitem.icon"));
      newFileButton.setTooltip(new Tooltip(handler.getString("jfx.gui.toolbar.newfile.tooltip")));
   .........
     ToolBar filesToolbar = new ToolBar(newFileButton, openFileButton, separator, saveFileButton, saveasFileButton);
  
    Platform.runLater(() -&gt; {
          toolbarContainer.getChildren().add(toolbar);
      });
  </code></pre>
  
  <p>Any idea what could be causing this?</p>