In a portlet containing the ckeditor wysiwyg such as the web content display, go to the 'add web content' page.
On this page go to the 'advanced' view that shows the additional 'structure', 'template' and 'schedule' options.
Reduce the browser window size to something like 1280px wide and maximise the 'schedule' options area - this reduces the width of the wysiwyg area.
Now click on the youtube button - not all of the youtube popup window is visible.
The user can click and drag the top title area of the popup to see the full window but the user can't use the scrollbar within the portlet to view the full popup window.