This is a known bug.
The only solution (for now) is to change the GTK theme in the KDE settings β Application Appearance.
They say (the error was filed somewhere), the problem is with the topic itself, but it's hard for me to believe ... Probably you have a set of oxygen gtk, change it, for example, to Raleigh, this will work.
Remember to restart monodevelop; -)
I like the oxygen theme, I find it the most annoying problem with md-4.0 ..
Edit: http://mono.1490590.n4.nabble.com/Text-editor-problem-with-monodevelop-4-0-td4658742.html - this is where I found the solution.
Piotr zierhoffer
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