M8: Grid auto-placement on an occupancy bitmap (#1593)

* M8: grid auto-placement on an occupancy bitmap (#1083)

Grid placement tested every cell of a child's span at every position it tried. The taken cells are now a bitmap: spans are checked a 64-bit word at a time, the search jumps past the blocking cell, a row's search ORs the span's rows once, and a column-anchored child looks on from where the last one with the same column and spans found room. Placements are unchanged (checked against the old search on random grids); a crafted 1000 x 1000 grid takes 0.3 ms instead of 192 ms.

* M8: grid occupancy stores rows only once a cell in them is taken

An empty or sparse grid no longer allocates the whole bitmap up front (CodeRabbit on #1593).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/auto-layout-parity.md

@@ -1,6 +1,6 @@
 # Auto layout and constraints parity
 
-> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-29 · **Change:** minor (Layout sidebar: Add auto layout (+) is available for groups and converts a single group to a frame in place, with mask order, flow-only inference and Fill sizing preserved; adding auto layout to an empty frame keeps its size (#369); `node.set` applies auto layout keys only to frames, with warnings (#603); core editor leftovers: auto gap handles, wrapped-line alignment; behaviour suite written from the public documentation, every rule's status; fixes and the missing settings it found) · **Target:** Figma Design (desktop app 126.10.7)
+> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-30 · **Change:** minor (grid auto-placement looks for free cells a 64-bit word at a time and skips what blocks it, with the same placements (#1083); Layout sidebar: Add auto layout (+) is available for groups and converts a single group to a frame in place, with mask order, flow-only inference and Fill sizing preserved; adding auto layout to an empty frame keeps its size (#369); `node.set` applies auto layout keys only to frames, with warnings (#603); core editor leftovers: auto gap handles, wrapped-line alignment; behaviour suite written from the public documentation, every rule's status; fixes and the missing settings it found) · **Target:** Figma Design (desktop app 126.10.7)
 
 Auto layout is the most used layout feature in Figma and the one where small differences (a
 hugging frame one pixel off, text wrapping at another width) break real files. **Breadth ~98%,
@@ -95,6 +95,7 @@ Status: **pass** (a case in the suite holds), **fixed** (the suite found a diver
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-30 | minor | M8, #1083: grid placement finds free cells on a bitmap, a word at a time, with the same placements; crafted grids of wide children no longer stall a layout pass. Numbers unchanged. |
 | 2026-10-29 | minor | M2 review fixes: conversion preserves mask runs and Fill sizing per axis; layout inference uses visible, non-absolute flow children; the sidebar hides Add for mixed selections containing existing auto layout. Regression coverage includes rendered mask pixels, visible-child positions, parent layout stability and the sidebar accessibility tree. Numbers unchanged |
 | 2026-10-29 | minor | M2: the Layout sidebar exposes Add auto layout (+) for groups. `layout.add` converts one selected group into a frame in place rather than wrapping it: same id, name and direct children; the layout arranges those children, and one undo restores the group. Sidebar, menu, shortcut and API calls all use the corrected command. This follows the contributor’s observed Figma behavior and the workflow familiar to users of similar design tools. Tests cover direct-child reflow, stable hierarchy, undo/redo and wrapping a multi-group selection. Numbers unchanged |
 | 2026-10-17 | minor | Community batch #753: adding auto layout to an empty frame keeps its size, Fixed, while a frame with content hugs it (#369); `node.set` applies auto layout keys to the frames among its targets and warns about the others, and a font style the family lacks warns with the nearest face (#603). Checked over MCP. Numbers unchanged |