How to set the text display area margin on textview.

[hustsii]

I want to render helper info by IBackgroundRenderer, it needs to move with the text when scrolling.
Sometimes I need to draw them at the left of the first char or after the last char.
I need the text display area smaller from the boundary rect where IBackgroundRenderer works.

The most direct way is to set the text display area margin on textview,
but I found the Margin property of textview does not work.
I struggled with it for 3days trying to modify the AvalonEdit but failed.