This is a very minor issue:
How to reproduce:
1. Enable Options > Visual Help > Folding enabled for suporting schemes
2. open a plain Text file (folding area is not visible as expected)
3. Select any highlighter that supports folding, e.g. PHP, the folding area is shown as expected
4. Select Plain Text Highlighter again
-> The folding area stays visible (not expected) until you restart PN
Surely no big deal and I don't see a real need to fix that, but I just wanted to report it...