Layout grids: Auto line width for columns and rows

Columns and rows can now set their line width to Auto: a width of 0 (or
typing the word "Auto" / "自动" in the field) draws a one-pixel hairline
down the band's middle, like the square grid, instead of a filled band.

- grid_lines: a non-stretched grid with size <= 0 emits a hairline at each
  band's middle rather than a zero/negative-width rect.
- canvas: a degenerate (zero-width or zero-height) grid rect is painted as a
  1px line; a filled band is still painted as a rectangle.
- number_field: a field whose placeholder is "Auto" also accepts the word
  "Auto"/"自动", mapping it to 0.
- right: the grid size field shows "Auto" and the align menu no longer
  forces a 60px band on a zero width.
- props: document the Auto semantics of LayoutGrid::size.
- ROADMAP.md / docs/ui-parity.md: note the change.
- layout tests: regression test for the Auto hairline.

docs/ui-parity.md

@@ -1,6 +1,6 @@
 # UI and interaction parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-18 · **Change:** minor (a rename selects the whole name, so typing replaces it; Escape or Enter after a pen path stays on the same vector (#760); IME composition kept on through idle frames of a text edit, candidates at the caret (#747); Selection colors for mixed selections and smart-selection spacing (#753); submenus with a hover delay and a safe triangle; the Actions palette's tabs, ranked search, highlight and recents; menu items that ask for a value; editor fixes from the community batches; fields select their text on focus; Typography spacing icons support scrubbing (#373); CSS colour names in hex fields; the Small caps sample in Type settings drawn from capitals; recent files as cards; vector edit mode: hovering, hatching, selecting and dragging filled regions; home screen: Templates tab, first-run templates and Getting started tips; number fields: scrub rates, arrow steps, arithmetic with brackets, relative edits of mixed values; outline mode's hidden layers and object bounds; Hide when publishing; cursors per tool and modifier; panel widths; dark pages for files made in the dark theme; drag threshold and double-clicks into text in groups; rotation origin on Alt+R and placing with Enter: no documented shortcut missing; keyboard shortcut parity suite and bindings; cursor chat and spotlight in live sessions; layers and assets panels behaviour suite and fixes; vector multi-edit and the vector edit tail; canvas navigation, guides and grids behaviour suite and fixes; shapes and vector behaviour suite and fixes; paint and effects behaviour suite and fixes; frames, groups, sections and masks behaviour suite and fixes; layers panel keyboard: F2 rename, Shift+F10 context menu, F8 box selection; selection and transform behaviour suite and fixes; Display P3: gamut warning and soft-proof (true P3 blocked upstream); several windows; boolean hit testing on the result, group drags, frame outlines and slices; outlines for the selection, mask outlines; arc fields, live boolean editing, shape builder on strokes, one-sided widths) · **Target:** Figma Design UI3 (desktop app 126.10.7)
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-19 · **Change:** minor (a rename selects the whole name, so typing replaces it; Escape or Enter after a pen path stays on the same vector (#760); IME composition kept on through idle frames of a text edit, candidates at the caret (#747); Selection colors for mixed selections and smart-selection spacing (#753); submenus with a hover delay and a safe triangle; the Actions palette's tabs, ranked search, highlight and recents; menu items that ask for a value; editor fixes from the community batches; fields select their text on focus; Typography spacing icons support scrubbing (#373); CSS colour names in hex fields; the Small caps sample in Type settings drawn from capitals; recent files as cards; vector edit mode: hovering, hatching, selecting and dragging filled regions; home screen: Templates tab, first-run templates and Getting started tips; number fields: scrub rates, arrow steps, arithmetic with brackets, relative edits of mixed values; outline mode's hidden layers and object bounds; Hide when publishing; cursors per tool and modifier; panel widths; dark pages for files made in the dark theme; drag threshold and double-clicks into text in groups; rotation origin on Alt+R and placing with Enter: no documented shortcut missing; keyboard shortcut parity suite and bindings; cursor chat and spotlight in live sessions; layers and assets panels behaviour suite and fixes; vector multi-edit and the vector edit tail; canvas navigation, guides and grids behaviour suite and fixes; shapes and vector behaviour suite and fixes; paint and effects behaviour suite and fixes; frames, groups, sections and masks behaviour suite and fixes; layers panel keyboard: F2 rename, Shift+F10 context menu, F8 box selection; selection and transform behaviour suite and fixes; Display P3: gamut warning and soft-proof (true P3 blocked upstream); several windows; boolean hit testing on the result, group drags, frame outlines and slices; outlines for the selection, mask outlines; arc fields, live boolean editing, shape builder on strokes, one-sided widths) · **Target:** Figma Design UI3 (desktop app 126.10.7)
 
 Fine canvas control is what Figma users feel first: handles, snapping, nudging, modifier keys,
 drag behaviour, numeric precision. This checklist covers the tools, gestures and panels. Status:
@@ -105,7 +105,7 @@ interaction feel not yet compared.
 | Frames and sections drawn around layers take those fully inside; sections never drop into frames | done | not in auto layout frames |
 | Resizing a section leaves its contents in place | done | |
 | Masks: what a mask hides doesn't take clicks; a mask group's box is the mask's | done | |
-| Layout grids (Ctrl+G) | done | columns, rows and grid; stretch, left, centre, right; Auto count; several per frame; styles; not shown or snapped on a rotated frame; a nested frame's grids snap but only top-level frames draw them |
+| Layout grids (Ctrl+G) | done | columns, rows and grid; Auto line width (0 draws a one-pixel hairline); stretch, left, centre, right; Auto count; several per frame; styles; not shown or snapped on a rotated frame; a nested frame's grids snap but only top-level frames draw them |
 | Gamut warnings for Display P3 files | done | colour picker note; View ▸ Soft-proof sRGB stripes what sRGB can't show (the canvas itself is sRGB) |
 | Gradient and padding/gap handles on the canvas | done | |
 | Hover outline, selection colours, handle sizes | done | measured from UI3 screenshots |
@@ -172,6 +172,7 @@ in the table. `crates/ui-egui/src/shortcut_keys_tests.rs` drives the keys a look
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-19 | minor | Layout grids: columns and rows take an Auto line width (0 or “Auto”), drawing a one-pixel hairline down each band's middle; the width field shows “Auto” and typing it clears the width. The align menu keeps a zero width as Auto instead of forcing 60 px. Regression test added. |
 | 2026-10-18 | minor | A rename (layers double-click, F2, File ▸ Rename, a page) selects the whole name in the frame its field first draws, so the first key replaces the name instead of being appended. Checked in the app on Windows over the control channel |
 | 2026-10-17 | minor | Escape or Enter while the pen draws ends the path and keeps its vector in edit mode with the pen, so the next path is drawn into the same layer (unconnected); another Escape leaves edit mode (#760). Entering edit mode on a layer that already is a vector no longer records an empty undo step. Numbers unchanged |
 | 2026-10-17 | minor | Text layers keep the platform IME on for every frame of an edit, not only frames with input (an idle frame turned it off and macOS dropped each composition, #747); the candidate window opens at the caret instead of the canvas corner; keys pressed while composing go to the IME |