Using the visual studio,when the mouse over the code folding button,the code including region line will become thick.
look at the attachment.
How to realize for the RadSyntaxEditor?
The RadSyntaxEditor control does not have support for this requirement. To achieve it, you can try to introduce custom logic for coloring the elements that represent the folding taggers.
After further examining this requirement, implementing it would not be possible due to the current implementation of the folding taggers. Due to this, I logged a new feature request in our feedback portal to add support for this inquiry. You can vote for it, as well as follow it, in order to get notified via e-mail when its status gets changed.
The feature request can be found at the following link:
SyntaxEditor: Add API for customizing the folding region lines and their respective regions (telerik.com)
I hope the provided information will be of help to you.