Leaving fullscreen mode does not make side-by-side view disappear

**Describe the bug**

If you are in fullscreen mode with side-by-side view and you leave fullscreen mode, the side-by-side previsualization stays on half the page

**To Reproduce**
Steps to reproduce the behavior:

1. With `sideBySideFullscreen` set to `false`
1. Go fullscreen mode
2. Activate side-by-side view
3. Leave fullscreen mode

**Expected behavior**
Side-by-side view should either disappear or be restricted to the editor

**Screenshots**

** Version information **
- OS: Ubuntu Budige 20.04
- Browser: Firefox 85
- EasyMDE version: 2.14.0

**Additional context**
Add any other context about the problem here.

Start by reproducing the fullscreen and side-by-side sequence with sideBySideFullscreen set to false, then trace the fullscreen and side-by-side state handling. Done means leaving fullscreen removes the side-by-side preview or restricts it to the editor, with the reported behavior covered by a regression check.

Written by the indexing model from the issue text.