sparkles:ui layout — Feature Requirements (LAY)

Status: decided · Date: 2026-07-29 · Scope: the layout level of sparkles:ui — the model, its units, the measure protocol, the sizing vocabulary, clipping and scroll, alignment, track sizing and text wrapping. This page is the decision record the UI-layout research catalog was gathered to settle.

IMPORTANT

The catalog deliberately states no verdict — its index notes that a comparison page "may be added here as the catalog matures". This page is that verdict. Requirements below cite the catalog as evidence; the choices are made here.

Design & rationale

The verdict

Keep the box-flow engine; add an orientation-aware measure protocol and a clip primitive; keep every unit an integer.

The existing two-pass engine (bottom-up intrinsic measure, top-down place) is already the family the survey recommends. What it lacks is not a different algorithm but three specific capabilities: a measure pass that knows the available width, flexible sizing that distributes leftover space, and clipping.

The surveyed families

Family	Representatives	Verdict
Box-flow, two-pass per axis	Clay	Adopted. Already our shape; O(n); allocation-free; its fit/grow/fixed/percent + min/max vocabulary is the catalog's first taxonomy row verbatim.
Flexbox	Yoga, Taffy, Stretch, CSS	Vocabulary adopted, algorithm rejected. A faithful implementation is 5–10 KLoC, and float coordinates are a documented terminal bug class (off-by-one cells on resize).
Constraints down, sizes up	Flutter, Compose, SwiftUI	Measure protocol adopted, engine rejected. Its intrinsics idea is what we need; adopting it wholesale would delete the bottom-up pass that grow sizing requires.
Constraint solvers	Cassowary, Kiwi, Auto Layout	Rejected. Thousands of lines of meticulous code, notoriously poor debuggability; the catalog's own conclusion is that for tree-shaped terminal UIs box-flow is right.
Classic retained two-pass	WPF, Qt, GTK, Swing/MiG, Tk	One idea adopted: GTK's orientation-aware measure(orientation, for_size), which the catalog calls rare among widget toolkits. Star sizing informs the track sizer.
Immediate-mode	Dear ImGui, egui	Rejected as a model (cursor-based layout fights responsive design), but egui's orthogonal alignment toggles inform LAY8.
Tiling window managers	i3/sway, xmonad	Two ideas adopted: proportional sizing as the default for content areas, and an introspectable serialized tree as a debugging tool (LAY12).
Line breaking	Knuth–Plass	Optional strategy, not architecture — see LAY10.

Why the measure signature is the crux

Today's measure takes no width, so a node cannot size against the space it will actually get. That single gap is why text wrapping was hoisted out of layout and into the view, against a hardcoded column limit. The width↔layout cycle ("wrapping needs a width; the width comes from layout") must be broken by ordering, not iteration: measure the main axis, allocate a width, then measure the cross axis parameterised by that width, at which point a text node runs its line breaker. That is GTK's protocol, and Flutter's minIntrinsicHeight(width) is the same idea under another name.

Why integers, everywhere

Two independent catalog entries report float-derived off-by-one cell errors as a recurring, shipped bug class in terminal backends. Because we hand-write the engine, the whole class is avoidable by never admitting a float: leftover space is distributed with divmod plus explicit remainder assignment, so the parts sum exactly to the whole at every width. Sub-pixel positioning remains available to pixel backends, which own the cells→device mapping (LAY3).

Layout model & units (LAY1–LAY3)

ID	Requirement	Status	Traces to
LAY1	Layout must turn a widget tree into one Frame per node — an absolute rectangle — via alternating bottom-up measure and top-down allocate passes (per axis, so four in total under LAY4), all @safe pure nothrow, in O(n) over the node count.	full (aa35bc26)	layout.d layout — naturalWidth/allocWidth/naturalHeight/place
LAY2	The layout model must be box-flow (the Clay family), selected from the UI-layout catalog; the flexbox vocabulary is adopted without the flexbox algorithm, and no constraint solver is used.	full (this page)	catalog verdict above; layout.d header
LAY3	Every coordinate and extent in the engine must be an integer in abstract cells. No floating-point value may enter layout. Mapping cells to device units (pixels, CSS lengths) is the canvas's responsibility, so a pixel backend may still position sub-pixel.	full (aa35bc26)	geometry.d Point/Size; canvas cellW/cellH

Measure protocol (LAY4–LAY5)

ID	Requirement	Status	Traces to
LAY4	measure must be orientation-aware and parameterised by the other axis — measure(axis, forSize) — so a node's extent on one axis may depend on its allocated extent on the other. The root receives the viewport. This is what resolves content-dependent sizing (text wrapping, height-for-width) by ordering rather than iteration.	full (aa35bc26)	layout.d — four passes: natural width → width allocation → height-for-width → height allocation + place
LAY5	Text measurement must be injected through the canvas, not hardcoded in the engine, and must be grapheme-correct (wide and combining characters counted properly). The measurer receives a candidate width so it can report wrapped extent; it must be assumed hot and cached.	partial	layout.d isTextMeasure/CellMeasure (the seam, aa35bc26); wiring each backend's grapheme-aware measurer through it is the M7 backend work

NOTE

The engine now measures through an injected isTextMeasure value (defaulting to CellMeasure, one column per codepoint), so the seam exists — but no backend passes its grapheme-aware measurer yet, and canvas.measure still has no caller. Until M7 wires it, wide characters overflow their boxes on the TUI.

Sizing (LAY6)

ID	Requirement	Status	Traces to
LAY6	The sizing vocabulary is fit / grow / fixed / percent with min/max clamps. percent is a fraction of the parent's content size. grow siblings share leftover space by weight; distribution must be integer-exact — q = leftover / weight; r = leftover % weight, with the r extra cells handed to the first r growers in order — so the parts sum exactly to the whole. On overflow, children shrink toward min.	full (aa35bc26)	geometry.d SizeSpec; layout.d distributeMain

NOTE

Overflow reclaim is proportional to each child's slack above min (integer divmod again) and is skipped on a clipped axis — a viewport's content is meant to overflow (LAY7), so it scrolls instead of being crushed.

Clipping, scroll & viewport (LAY7)

ID	Requirement	Status	Traces to
LAY7	A container must be able to clip and scroll its subtree: a per-node clip (per axis) plus a child offset, which place subtracts from child origins and which emits scissor push/pop operations around the subtree in the display list. Scroll offset is interaction state and lives in a state machine, not in the widget arena. It must map to overflow on the HTML target.	full (9952b61c)	Widget.clipX/clipY/childOffset; display_list pushClip/popClip + culling; CellGrid/hue GridCanvas clip stacks; interp/html overflow

Alignment (LAY8)

ID	Requirement	Status	Traces to
LAY8	Containers must support one alignment value per axis, offsetting each child within its allocated band. Distribution effects (space-between and friends) are produced by inserting grow spacer children, not by a separate property. Cross-axis stretch stays a per-child flag.	full (cf0131e4)	Widget.alignX/alignY (Alignment); interp/html align-items/justify-content

Track sizing (LAY9)

ID	Requirement	Status	Traces to
LAY9	Tabular layout must be served by a one-dimensional track sizer — per track auto / fixed(n) / fr(w) / minmax(a,b), resolved as "measure auto tracks, subtract fixed and gaps, divide the remainder among fr by weight with remainder distribution" — with cell spans. It must emit grid-template-columns on the HTML target. Row extents come from LAY4's cross-axis measure.	partial	the sizer + spans + grid-template-columns emission shipped (a9f7698e, tracks.d); the table view adopts it in M3c

Text wrapping (LAY10)

ID	Requirement	Status	Traces to
LAY10	Line breaking must sit behind a strategy seam — greedy by default, with a balanced (rigid-glue Knuth–Plass) variant selectable — invoked from the cross-axis measure with the allocated width. Hang indent is expressed as a first-line width delta. No wrap width may be hardcoded in a view.	full (407bce58)	sparkles.ui.wrap (TextWrap greedy/balanced + wrapSpans for styled runs, hang indent), invoked from the height-for-width pass. The twoslash docs packer is deleted — its 56 columns survive only as a Widget.width.max style metric the engine wraps against

NOTE

Knuth–Plass is deliberately not architecture. The catalog notes it degenerates to greedy at table-cell widths and that a monospace grid has no stretchable glue, so ragged-right greedy is the natural default; the balanced variant is a quality upgrade for full-width prose, selectable per call.

Ergonomics (LAY11–LAY13)

ID	Requirement	Status	Traces to
LAY11	Nodes must support tri-state visibility — visible / hidden (occupies space) / collapsed (removed from flow) — so chrome can toggle without a layout jump. It maps to visibility and display:none on HTML.	full (cf0131e4)	Widget.visibility (Visibility); layout.d collapsed-skip; display_list hidden-skip
LAY12	The laid-out tree must be introspectable — serializable to a readable form (nodes, kinds, slots, resolved rects) for debugging, via the existing pretty-printing machinery.	full (cf0131e4)	layout.d dumpTree
LAY13	Measured extents should be cached keyed on (node, axis, forSize), before any dirty-tracking or incremental-relayout mechanism is considered.	not started	proposed measure cache

Explicitly out of scope

Each exclusion is a decision with evidence, recorded so it is not silently revisited:

Not implemented	Why
margin	Clay has shipped for years without it; Flutter deliberately replaced it with gutter widgets; CSS margin collapsing is the most surprising part of normal flow. gap + padding cover our cases. The currently-inert Widget.margin field is deleted, not left unread.
Constraint solvers	Implementation cost and debuggability; the catalog concludes box-flow is the right tool where layout maps onto a tree, which ours does.
Full CSS Grid	The auto-placement and staged track-resolution algorithms run to pages of normative prose; essentially only browser-grade engines implement it. LAY9 ships the useful subset and emits grid CSS.
flex-wrap / box wrapping	Absent from Clay, Tk and ConstraintLayout alike. Our text wraps; our boxes do not need to. Text wrapping is not flex wrapping.
The full flexbox alignment matrix	justify-content × align-items × align-content × flex-wrap produces surprising combinations; one value per axis plus spacers covers the same ground.
Baseline alignment on cell targets	Irrelevant on a fixed grid — all cell text shares a baseline. Revisit only if a pixel backend grows mixed font sizes.
Eager intrinsic-width queries	Documented as expensive by the toolkits that offer them, and catastrophic inside large columns. LAY4's forSize obtains the same answers within the normal pass.
Floating-point coordinates	Two independent catalog entries report float-derived off-by-one cell drift as a shipped bug class in terminal backends.
Lazy off-screen materialisation	Genuinely valuable at very large item counts, but a second-order optimisation on top of LAY7. Measure before building it.

Milestones

Milestone	Scope	Status	Requirements
L0	Decision record (this page) — model, units, exclusions	decided	LAY2, LAY3
L1	Orientation-aware measure + injected grapheme-correct text measurement	partial (aa35bc26; M7 wires the backend measurers)	LAY4, LAY5
L2	Integer-exact grow/percent distribution	full (aa35bc26)	LAY6
L3	Clip, child offset and scissor emission	full (9952b61c)	LAY7
L4	Alignment, tri-state visibility, tree dump; delete margin	full (cf0131e4; margin deleted in aa35bc26)	LAY8, LAY11, LAY12
L5	Track sizer + wrapping strategy seam	partial (fcd2464a, a9f7698e; view adoption in M3c/M6)	LAY9, LAY10
L6	Measure caching (and dirty relayout only if measurement demands it)	not started	LAY13

Module coverage

Source file	Requirements
libs/ui/src/sparkles/ui/layout.d	LAY1, LAY2, LAY4–LAY8, LAY11, LAY12, LAY13
libs/ui/src/sparkles/ui/geometry.d	LAY3, LAY6 (SizeSpec); Rect.intersection behind LAY7
libs/ui/src/sparkles/ui/wrap.d	LAY10 (TextWrap, greedy + balanced; landed here rather than the once-proposed base.text.wrap — the measurer is caller-supplied, so the breaker is the toolkit's)
libs/ui/src/sparkles/ui/tracks.d	LAY9 (TrackSpec, resolveTracks, applySpans, writeGridTemplate)
libs/ui/src/sparkles/ui/canvas.d	LAY5 (the measure primitive); pushClip/popClip ops (LAY7)
libs/ui/src/sparkles/ui/display_list.d	LAY7 (scissor emission + culling)

Relationship to existing specs

Piece	Role in layout
UI-layout catalog	the evidence base this page decides from (LAY2)
backends.md TGT	the cells→device mapping (LAY3) and the measure primitive (LAY5)
widgets.md WGT	the tree layout consumes; the table/scroll widgets driving LAY7/LAY9
state-machines.md STM	scroll offset, which LAY7 reads but does not own
sparkles.base.text (wrap, grapheme)	the wrapping and width primitives behind LAY5/LAY10

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