Function visible_source_lines 

pub fn visible_source_lines(
    fold_index: &FoldIndex,
    scroll: u32,
    height: u32,
    total_lines: u32,
) -> Vec<u32>

Number of buffer lines collapsed onto the visible head row of the closed fold (start_line, end_line) — the count both renderers show in the ⋯ N lines fold summary. Walks forward from end_line + 1 through any sibling closed folds whose heading is itself hidden by the region already collapsed (start < probe <= end), so overlapping folds (e.g. (1,3)+(3,5) from foldmethod=indent) report their combined span. Folds that merely ABUT — the next fold starts at end + 1, the first visible line after this one — are NOT chained: that fold has an on-screen heading and is a separate fold with its own summary. Shared by the TUI and GPUI renderers so the count stays identical. The source lines a pane actually shows, in paint order: walk from scroll collecting lines until height VISIBLE ones are gathered, skipping every line hidden inside a closed fold and stepping over a closed fold’s body in one jump (its head row stands for the whole range).

The bound is display rows, not source lines, and that is the entire point. Taking [scroll, scroll + height) and filtering the folded lines out afterwards looks equivalent and is not: each collapsed line still spends one of the window’s slots, so the pane under-fills the moment a fold closes and every line past the window — further headings, the cursor — is never considered at all. GPUI shipped that bug (2026-08-09); the TUI had this walk inline from the start. It lives here now so the two peers cannot drift again.

Soft-wrap is deliberately NOT accounted for: a wrapped line’s extra segments are display rows the renderers add downstream, so this can over-collect when wrapping is on. Both renderers cap their row budget while emitting (shaped_text.len() >= viewport_height / out.len() >= height), which is where wrap is paid for.

total_lines is CONTENT space — lattice_core::Buffer::content_line_count, never ropey’s raw [lattice_core::Buffer::line_count]. This function decides which source lines get a row, so the raw count’s phantom trailing line (present for every rope ending in \n, i.e. every normal file) becomes a phantom painted row: the numbered blank line 220 of a 219-line file that CV.2 was reported for. Both renderers call through here, so passing the wrong space in either one reintroduces it in that peer alone.