Width of TinyMCE iframe narrower than bounding area

Each time, on a browser with cookies and cache completely cleared:
With Clean - for small screens, wide screens, and a few screen sizes in between:
With Standard - for decent sized screens (1024+) and wide screens:

• load a page with Tiny MCE, fill it with content (also check multiple choice questions on quiz)

• ensure the scrollbar appears in the right place
• ensure the editor appears on the page correctly
• ensure the editor takes up the same area as the filepicker below it
• ensure you can still resize the editor smaller