Function active_indent_guide 

pub fn active_indent_guide(
    caret_row: u32,
    line_count: u32,
    rows: Range<u32>,
    level_at: impl Fn(u32) -> u32,
) -> Option<(u32, u32, u32)>

Which indent guide is active (highlighted) for the caret’s line.

Returns (indent_level, start_row, end_row) inclusive, or None when no guide is active (the caret sits at level 0 with no deeper neighbour). level_at(row) yields a row’s indent level (content = ceil, blank = interpolated, matching indent_guide_level); rows are 0-based, line_count the total.

The scope special-cases are the whole point: on a line that opens a deeper scope (the next line is more indented) the first child level is highlighted — the opener’s own level + 1, i.e. one tab stop in from the opener, regardless of how deep the body actually sits — and symmetrically on a line that closes one (the line above is more indented). Off a brace line it is just the caret line’s level. The chosen level is then extended over the contiguous run of rows at least that deep.

Pinning the highlight to initial + 1 (not the body’s real level) is deliberate: an over-indented body (opener at col 4, body at col 16) still lights the guide one stop inside the opener — cell 4 — rather than the body’s own deep guide. For uniformly-indented code the jump is one level, so initial + 1 already equals the body level.

rows bounds the outward walk: the guide only paints inside the caller’s viewport, so the extent search never needs to leave that clamp (the widget passes its window ± slack; tests pass 0..line_count for the unbounded behaviour). A caret row outside rows yields None — an off-screen caret’s scope highlight is not painted anyway.