Opening "Create email template" for a Form can be erroneous

Description

Steps to reproduce
1. Go to the settings of a Form container.
2. Click "Send email after form submission".
3. At the right bottom corner of the TinyMCE editor, click the arrow facing down and to the right and drag it as far to the right as you can.
4. Notice that you can't resize it again.

Expected:
You should be able to resize it.

Actual:
TinyMCE editor window gets stuck.