Scroll bars disappear after using full screen option in TinyMCE

Description

Steps to reproduce:
1. Go to CMS Edit mode, open the Alloy Plan page.
2. At the Main body field's editor, click the Fullscreen button.
3. Click the back button to exit Fullscreen mode.
4. Open the Alloy Plan page again.
5. Choose the Content tab.

Expected:
There should be a vertical scroll bar on the right, for the editing window.

Actual:
The vertical scroll bar is disabled and hidden.