The skin switches to the mobile one after adding an image in WYSIWYG on IE9

Steps to reproduce:

1. Edit a page in WYSIWYG
2. Click on "Image > Attached Image..."
3. Click on "Upload new image"
4. Click Select
5. Select an image from your computer
6. Click "Insert Image"

After the image is successfully attached the skin is changed to the mobile one, even though the width & height of the screen haven't changed.

Doesn't reproduce on IE11, Chrome & Firefox.