Line layout prototype

Question

Can a terminal reproduce a TUI renderer’s layout at any width from the line-level attributes proposed by RFC 0003, given content rendered once without width breaks?

layout.ts is a pure layout engine for the proposed text/lines content: a function from lines and a column count to display rows. It is the part of a terminal-side implementation that decides rows; it does not render, track history, or speak the wire protocol. LayoutLine is an experimental local type, not a protocol schema.

Rules implemented

  • word wrap (default): a break opportunity follows each run of spaces and lies between two adjacent wide characters. Spaces at a break are dropped from display but remain logical text, so row offsets skip them. A word wider than the row starts a new row and then breaks between cells. Rows never end with displayed spaces. This matches the wrapping rule of Pi 0.87.1’s wrapTextWithAnsi.

  • char wrap: breaks between cells.

  • continuation: displayed at the start of every soft-wrapped row, narrowing that row’s width; omitted when it would leave no width.

  • fill: one grapheme repeated across the full width, in whole cells.

Each row records its source line, whether it is soft, its prefix width, and the logical scalar offset of every text cell. projection.ts uses them to map cells to a Block’s logical text for copy (no prefixes, no soft breaks, dropped spaces kept) and logical search matches back to cells (checks).

cellWidth is an experimental table: two cells for East Asian wide and fullwidth ranges and common emoji, one otherwise.

Evidence

  • Unit checks: nine rules above, including CJK breaking, prefix omission, and per-line offsets.

  • Pi comparison, 2026-10-19: Pi’s wide render of the wrap sample, converted to text/lines by an experimental reader, laid out at 36 and 60 columns, equals Pi’s own render at those widths row for row, styles included. At 100 columns the only difference is the rule: Pi caps it at 80 cells, the fill line spans the width. The cell reflow of the same wide render keeps 13 of Pi’s 29 or 24 rows (layout experiment).

  • Width sweep, 2026-10-20: a second sample (CJK_NESTED_SAMPLE: Chinese prose, mixed Chinese and English, nested bullet and numbered lists, a Chinese quote) equals Pi’s render at every width from 20 to 100. The English sample differs only by the rule cap above 80 and, at 21, 39, 58, and 74 columns, by Pi keeping a space at the start of a row when the break follows a styled span (likely because the wrapper’s whitespace test, token.trim() === "", sees the style reset in that token; not traced further); the engine drops break spaces as RFC 0003 specifies.

Limits

One renderer and two samples. The reader recovers layout intent from the rendered shapes of the samples’ constructs; tables, CJK punctuation rules beyond Pi’s, and other renderers have not been compared. The cell-width table is a small subset of Unicode East Asian Width.