Function split_render_entry 

pub fn split_render_entry(
    entry: Option<&SplitDiffEntry>,
    content_width: usize,
    lang_hint: &str,
    side: Side,
    gutter_width: usize,
    gutter_tint: GutterTint,
    context: &ViewContext,
) -> Frame

GutterTint::Diff extends the diff background and foreground through the line-number gutter of changed rows, forming a clean color block. Inline previews use GutterTint::Neutral so indenting the frame cannot pick up a diff background (see Line::prepend).