# iris: one `draw` that reports a size

Iris, 2026-09-04:

> I don't like that widgets need both a draw and size functions. I'd much
> rather them have a single draw that reports a size, and if it needs to be
> moved then that can be done after the fact efficiently, or resized just
> done after as well. This should be done efficiently like everything else
> tries to do right now.

**Implemented 2026-09-04.** Every widget was migrated in one change; none
kept `desired_width`/`desired_height`. What is kept below is the design as
it stands, the five corrections implementation forced (read those before
touching `Aligned`, `Sized`, `MaxSize`, `Scroll` or the move-slot lifecycle
in `render_state.rs` -- each is a real bug the first draft would have
reproduced), and the two later additions that build on it. The
pre-implementation framing -- what the old trait looked like, the checklist
the design had to answer, the migration list, the pass conditions and the
"copy this into the design log" note -- was deleted on 2026-09-08,
having been carried out.

## Design

### 1. The new `Widget` trait

```rust
pub trait Widget: Any {
    fn draw(&mut self, painter: &mut Painter) -> Size;

    fn size_hint(&self, axis: Axis) -> Option<Len> { None }

    fn is_size_independent(&self) -> bool {
        false
    }
}
```

No `available` parameter: `Painter` already carries the region the parent
handed down (`Painter::region()`, `core/src/ui/painter.rs:137`) and already
exposes the pixel-resolved form (`px_size()`, `:156`) and the output surface
size (`output_size()`, `:152`). Passing it again would be the same value
under a second name. `desired_width`/`desired_height` (`core/src/widget/mod.rs:20-21`)
and `WidgetAxisFns::desired_len` (`:24-35`) are deleted outright — not
deprecated, not kept as a fallback — because a widget that implements both
`draw` and `desired_*` for the same thing is exactly the "two names for one
concept" the code rules call out, and it is what today's `Span::desired_ortho`
(`iris/src/widget/position/span.rs:98-152`) already complains about in its
own comment: "this literally copies draw so that the lengths are correctly
set in the context, which makes this slow and not cool." Folding sizing into
`draw` deletes that duplicate simulation, not just moves it.

`size_hint` is not a second layout pass. It is an optional exact answer for
an axis the widget declares without painter context or child access. A
lying hint fails a debug assertion when the widget is drawn.

### 2. Move: O(1) per moved subtree, via a per-widget offset chain

**What exists today, and why it is not O(1).** `UiRenderState::mov`
(`core/src/ui/render_state.rs:156-168`) fires when a widget's region keeps
its *size* but changes *position* (`draw_inner`, `:85-100`:
`active.region.size() == region.size()` after excluding the exact-match
case). It rewrites every primitive's `region` field via
`Primitives::region_mut` (`core/src/render/primitive.rs:176-179`) for the
widget's own primitives, then recurses into every child — O(primitives in
the subtree). Both call sites that trigger it today, `Scroll::draw`
(`iris/src/widget/position/scroll.rs:29-31`) and `Offset::draw`
(`iris/src/widget/position/offset.rs:9-11`), are "translate this subtree by
an abs pixel amount, `rel` framing unchanged" — a transcript scroll
re-touches every glyph in every visible row, every frame of the drag, and
I3's target is 800 rows on screen.

**Recommendation: a per-widget offset slot forming a parent-linked chain,
resolved in the vertex shader.**

- `UiData` (`core/src/ui/mod.rs:14-20`) gains
  `pub move_offsets: TrackedArena<MoveOffset, u32>`, the same arena shape
  already used for `masks: TrackedArena<Mask, u32>` on the line above it.
- `render/data.rs` gains `pub struct MoveOffset { pub delta: [f32; 2], pub
  parent: u32 }` (`Pod`/`Zeroable`, `parent = u32::MAX` = "no ancestor,
  add nothing more"). A pure abs-pixel translation, not a general
  `UiRegion` remap — sufficient for every existing call site (above).
- `PrimitiveInstance` (`render/data.rs:11-18`) gains `pub move_idx: u32`,
  a vertex attribute at `@location(7)` beside `mask_idx` at `6` — the same
  kind of per-instance handle.
- `ActiveData` (`core/src/ui/active.rs`) gains `pub move_slot: MoveIdx`,
  assigned **when the widget is first drawn** (`draw_inner`, beside
  `active.insert`), with `parent` = the drawing widget's parent's slot.
  `Painter` threads a `move_slot` field down exactly as it already threads
  `mask` and `layer` (`painter.rs:9-20`), so a freshly-drawn descendant is
  correct from its first frame — nothing is ever retrofitted onto an
  already-active primitive. An unmoved widget's slot just stays `[0, 0]`.
- `Painter::primitive_at` (`painter.rs:23-38`) writes `move_idx:
  self.move_slot`, matching how it already writes `mask_idx: self.mask`.
- `mov(id, delta)` becomes: look up `id`'s slot, write
  `move_offsets[slot].delta += delta`. One write — no primitive touched, no
  recursion, since descendants already reference this slot transitively.
- A container may also retain one optional **child-coordinate slot** between
  its own slot and every direct child's slot. `Painter::set_child_offset`
  creates that boundary before the first child is drawn and can update it
  after measuring a child on later redraws. The container's own primitives,
  hit region and mask stay fixed; its whole child subtree moves through one
  write and every existing GPU, hit-test and accessibility chain sees the
  same result. `LazySpan` uses this while still walking visible rows for
  virtualisation: row boxes stay in stable local coordinates and the shared
  boundary carries the changing screen translation.
- `shader.wgsl`'s vertex stage, after computing `top_left`/`bot_right` in
  pixels (after `:106`, before the clip-space divide at `:113`), walks
  `move_idx → move_offsets[i].parent` for a bounded number of steps (a
  small constant, e.g. 16, with a CPU-side debug assertion that no chain
  exceeds it), summing `delta` into both corners. Cost is O(chain depth),
  paid every frame regardless of whether anything moved — negligible next
  to the per-fragment texture sampling TEXTURES.md already measures this
  GPU as not bound by.

**Why the chain, not the flatter thing first proposed.** Iris's own
phrasing — "every instance carries an index into a small per-widget offset
buffer" — describes a flat table: one slot per subtree *declared* movable,
no parent link. It breaks the moment two such subtrees nest — a row inside
a scrolling list, itself later given its own animated offset (a
swipe-to-delete mid-scroll) — because the row's primitives would have to
pick one slot and lose the other's contribution. The chain costs one extra
field and a bounded shader loop in exchange for no such gap, and since
every `ActiveData` gets a slot unconditionally rather than lazily, it costs
no more at the common depth of one than the flat version would.

**Against `region_mut` as the steady-state mechanism**: rejected for being
O(primitives in the subtree) — the cost this section removes — but kept
for a resize that changes a region's `rel` component (a genuine reflow,
§3) and for a size-independent widget's resize (§3), where the content's
shape doesn't change and one field write already suffices.

Provisional layout can still write an instance at an intermediate position
and restore it before upload. `Primitives::set_instance` remembers the value
at the first write in a frame and clears the dirty bit when the final bytes
match it. The GPU therefore observes final layout state, not CPU-only
measurement work.

### 2b. Two more readers of "where is this widget," and masks

Moving the offset into the vertex shader means `ActiveData.region` is no
longer the on-screen truth once a widget has been moved — it is where the
widget was *drawn*, before any `move_offsets` delta. Two things read it as
if it still were, and both must move to a resolved query or they silently
answer with the pre-move position: a click landing on a scrolled row would
be routed to whatever used to be there, with nothing on screen to say so —
exactly the "wrong answer that looks like a right one" case the code rules
single out.

**Hit-testing.** `SensorUi::run_sensors` (`src/default/sense.rs:154-200`)
does the actual pointer routing, and line 170 is the read in question:
`let shape = self.active.get(id).unwrap().region;` (`self: &UiRenderState`),
immediately turned into pixels and tested against the cursor at `:171-172`.
Under this design that region must be resolved through the same chain the
GPU walks before it means anything. Add to `UiRenderState`:

```rust
/// `active[id].region`, corrected by every `move_offsets` delta between
/// `id` and the root — the CPU-side twin of the vertex shader's chain
/// walk, over the same arena, so the two cannot disagree about where a
/// widget is. O(chain depth), not O(primitives): a plain Rust loop over
/// `move_offsets`, bounded by the same constant the shader loop uses
/// (name it once, e.g. `render::MOVE_CHAIN_LIMIT`, and reference it from
/// the WGSL loop bound in a comment, since WGSL cannot `include!` a Rust
/// const across the language boundary).
pub fn resolved_region(&self, id: WidgetId) -> UiRegion;
```

`window_region` (`core/src/ui/render_state.rs:264-267`), the public
coordinate query already used outside hit-testing
(`src/default/attr.rs:15,17,70`, e.g. positioning one widget relative to
another's on-screen box), is reimplemented to call `resolved_region(id)`
before `.to_px(...)` instead of reading `.region` directly — one change
covers both call sites listed there. `sense.rs:170` changes to
`let shape = self.resolved_region(*id);`. Both are required the moment §2
lands, not an optional follow-up: an unmoved widget's chain is empty and
`resolved_region` costs one arena read to find that out, so there is no
version of this design where skipping the fix is a legitimate
optimization — it is a correctness gap, not a performance one.

**Masks.** `Painter::set_mask` (`core/src/ui/painter.rs:49-52`) bakes the
painter's *current* region into a `Mask` pushed onto
`masks: TrackedArena<Mask, u32>` (`core/src/ui/mod.rs:19`), and the
fragment shader clips every primitive against `masks[in.mask_idx]`'s raw
`rel`/`abs` fields, unaffected by any move (`shader.wgsl:147-157`). If the
widget that called `set_mask` — `Masked::draw`,
`iris/src/widget/mask.rs:7-11`, `painter.set_mask(painter.region()); ...` —
is itself later moved, its clip rectangle stays where it was drawn while
its content moves out from under it: a visibly wrong clip, immediately on
screen, not a latency question.

Fix: `Mask` (`core/src/render/data.rs:46-49`) gains `pub move_idx: u32`,
written from `Painter::set_mask` as `self.move_slot` — the identical slot
the mask-owning widget's own primitives already get (§2), not a second
mechanism. Resolution happens in the **fragment** shader, not the CPU, and
not the vertex shader either: `shader.wgsl`'s mask check (`:147-157`)
currently computes the mask's `top_left`/`bot_right` inline from
`masks[in.mask_idx]`; that computation is extended to walk the same
move-offset chain §2 added, via one shared function —

```wgsl
fn resolve_move(idx: u32) -> vec2<f32> { /* the bounded parent walk, used by both stages */ }
```

— called from `vs_main` for a primitive's own corners and from `fs_main`
for its mask's corners, so the walk is written once and the two stages
cannot drift apart (the sibling-rule from the code rules: one loop, not a
hand-copied second one in the other shader stage).

**Why the fragment shader, not a CPU-side mask rewrite at move time.** A
primitive's mask is frequently owned by a *different* widget than the
primitive itself — often several levels up a subtree, with its own,
independent move slot — so a primitive's resolved offset and its mask's
resolved offset are two different chain sums, both needed, and only the
fragment shader has both `in.move_idx` (this fragment's own chain) and
`in.mask_idx` (indirecting to a second, possibly unrelated chain) already
in hand per-fragment. Resolving mask regions on the CPU at move time would
mean, for every `mov()` call, walking forward to every mask instance the
moved widget's slot could affect and rewriting its raw region — exactly
the O(subtree) cost §2 exists to remove, just moved from primitives to
masks. The fragment shader already re-reads `masks[in.mask_idx]` every
frame (`:148`); one more arena read to resolve its chain costs nothing
extra in kind.

**The scroll-container case, checked rather than assumed.** A masked,
scrollable region is built as a `Masked` wrapping a `Scroll`
(`iris/src/widget/position/scroll.rs`, `iris/src/widget/mask.rs`) — the
viewport border is drawn (and `set_mask` called) by `Masked`, which is
never itself the target of `mov()`; only `Scroll`'s inner content is,
every frame the user drags. Because each widget's move slot is its own
(§2: assigned per `ActiveData`, not shared), `Masked`'s mask references
its own, stationary slot, while the scrolled content underneath references
a separate, deeper slot whose `parent` chain passes through — but does not
write to — the viewport's slot. Moving the content therefore never touches
the mask's resolved position, and the mask staying still while its content
slides past it is what this design already produces with no special case,
not an extra rule that had to be added for it.

### 3. Resize scope

A resize is "the region a widget's parent offers it changes such that the
widget's draw might produce different output" — as opposed to a move, which
by construction cannot (§2 is scoped to pure translation). Two independent
narrowings apply, and both are real, measured properties of the code as it
stands rather than new machinery:

**(a) A window resize does not, by itself, require touching most widgets.**
`shader.wgsl:105-106` recomputes every primitive's pixel position from
`window.dim` and the primitive's stored `rel`/`abs` pair *every frame,
already, on the GPU*. A widget laid out purely in `rel`/`abs` terms (no
call to `px_size()`, `output_size()`, or anything else that reads a
concrete pixel count) is therefore already correct after a resize with zero
CPU work — the shader did it. `UiRenderState::needs_redraw_all`
(`render_state.rs:229-231`) currently ignores this and redraws the entire
tree on every `resized`, which was the safe default while sizing and
drawing were two passes; it should be narrowed to only the widgets that
*do* read a concrete pixel value. Track this the same way `needs_redraw`
already tracks per-widget dirtiness (`Widgets::needs_redraw`,
`core/src/widget/widgets.rs:9`): a widget's `draw` call marks itself
pixel-dependent by calling through `Painter` methods that read
`output_size`/`px_size` (both already funnel through `Painter`, so the
marking is one line at each), and `resize()` (`render_state.rs:32-35`)
walks only that set instead of unconditionally setting `resized = true`
for a full `redraw_all`. This turns "every resize redraws everything" into
"every resize redraws what depends on pixels" — a real behavior change
beyond what was asked, so verify it against the I0b `pre_present_notify`
resize regression (that fix depended on `redraw_all`'s completeness)
before narrowing this.

**(b) A widget's `available` (its parent's offered region) can change
without the widget's *content* changing — this is what
`is_size_independent` (§1) answers.** When a container's own layout shifts
(a sibling grew or shrank, changing this widget's offered box), a widget
that returns `true` from `is_size_independent` is not redrawn: its
primitives are unaffected by size, only by placement, so the parent
either (i) issues a move (§2) if only position changed, or (ii) rewrites
the primitive's `region` fields directly via `region_mut` if the box
changed shape too (still O(primitives owned directly by this widget, not
its subtree, since a size-independent widget by definition has no
size-dependent descendants worth distinguishing — in practice this is
always a leaf: `Rect`, `Image`, a fixed glyph). A widget that returns
`false` (the default) is redrawn in full whenever `available` changes,
which is correct always, just not free.

**Size propagation goes both ways in the same frame.** A resized child first
walks upward through exactly the ancestors whose cached size changes. That
measurement pass gives each parent the new size but necessarily drew the
branch in its old boxes. As the recursion returns, `redraw_and_settle` revisits
those changed widgets from the outside in, after their parents have assigned
the final boxes. Otherwise a newly appended child can retain the provisional
(even inverted) region it was measured in until another update happens. The
downward work is confined to the branch that changed; unchanged descendants
still take `draw_inner`'s retained fast path. Each downward visit is exactly
one redraw, not another upward propagation: a wrapping child can have no fixed
point when an ancestor shrink-wraps it (a trailing space alternated between one
line in the offered width and two lines in its reported natural width). Feeding
that answer back into the same branch recursively overflowed Android's native
UI-thread stack before Rust could report a panic. A container that intends a
wrapping child to occupy its width declares that constraint explicitly; the
message composer does so on both sides of its vertical `ScrollArea`.

### 4. Wrapped text, and "needs child height before choosing width"

**Wrapped text is not a special case any more; it already reads as one
draw.** `TextView::render` (`iris/src/widget/text/mod.rs:57-76`) already
does exactly what single-draw asks for: it reads `ctx.px_size().x` as the
wrap width, shapes once, and memoizes the shaped layout keyed on that width
plus a changed-flag on the buffer and attrs (`:63-69`) — a second call with
the same width is a hash-map-style cache hit, not a re-shape. Under the new
trait this collapses `Text::draw`/`desired_width`/`desired_height`
(`text/mod.rs:133-147`, three functions) into one `Text::draw` that calls
`self.view.draw(painter)` once, which internally still calls `render`
once, hits its own cache, and returns the size it already computed. No
new caching is needed here; the two now-redundant call sites
(`desired_width`/`desired_height` each separately calling `render`) simply
disappear, which is a second `render` avoided per frame per text widget
that is being measured by a parent.

`Span` has no size-only pass. It first reads exact, context-free
`Widget::size_hint(axis)` values. It then draws unknown fixed children
forward from the current cursor, retaining what they paint. Once every
length is known, flexible space is allocated and `Painter::place` moves
each retained child into its final box. A child is redrawn only when that
box changes the size it was drawn for.

Hints are optional and affect cost, never correctness. `Sized` can report
its declared axis without inspecting its child, which covers the important
`.height(rest())` case. A debug assertion compares every hint with the
eventual `draw` result. Widgets whose answer depends on shaping or on a
child return `None`.

### 5. Caching and invalidation

`Cache.size` (`core/src/ui/cache.rs`) is **deleted, not replaced with an
equivalent** — the thing it memoized (a `desired_width`/`desired_height`
answer, independent of drawing) no longer exists as a separate query, so
there is nothing left to cache at that layer. What already provides "an
unchanged subtree costs nothing" is the check `draw_inner` performs before
touching a widget at all (`render_state.rs:85-90`): if the widget is active,
its region is unchanged, and it is not marked dirty, `draw_inner` returns
immediately — no `Painter` constructed, no primitive touched, no shader
work beyond what the GPU already redraws from the unchanged instance
buffer. That check is kept exactly as it is; it is the caching mechanism,
and it already operates at (id, region) granularity, which subsumes "(id,
available size)" once size *is* what a region change means.

What is added: `ActiveData` gains `pub size: Size` — the value `draw`
returned, stored the moment it is (`draw_inner`, alongside building the
`ActiveData` struct at `:134-143`). This is what a parent placing this
widget for a second frame without redrawing it (because nothing changed)
reads instead of recomputing — it replaces `Cache.size`'s role of "answer a
size question without a full draw" with "read the size of the last actual
draw," which is always available because `draw_inner`'s skip path is only
reachable once the widget has been drawn at least once. `Cache::remove`/
`Cache::clear` (`cache.rs:9-17`) are deleted with the type; `ActiveData`
already has an equivalent lifecycle (removed in `remove`/`remove_rec`,
`render_state.rs:171-198`, freed with the widget).

### 6. Before / after

**A leaf, `iris/src/widget/rect.rs`** — the size-independent case:

```rust
// before
impl Widget for Rect {
    fn draw(&mut self, painter: &mut Painter) {
        painter.primitive(RectPrimitive { color: self.color, radius: self.radius,
            thickness: self.thickness, inner_radius: self.inner_radius });
    }
    fn desired_width(&mut self, _: &mut SizeCtx) -> Len { Len::rest(1) }
    fn desired_height(&mut self, _: &mut SizeCtx) -> Len { Len::rest(1) }
}
```

```rust
// after
impl Widget for Rect {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        painter.primitive(RectPrimitive { color: self.color, radius: self.radius,
            thickness: self.thickness, inner_radius: self.inner_radius });
        Size::REST // fills whatever it was given -- used == available
    }
    fn is_size_independent(&self) -> bool { true } // content never depends on region size
}
```

**A container that needs the child's size before placing it,
`iris/src/widget/position/align.rs`**:

```rust
// before
impl Widget for Aligned {
    fn draw(&mut self, painter: &mut Painter) {
        let region = match self.align.tuple() {
            (Some(x), Some(y)) => painter.size(&self.inner).to_uivec2().align(RegionAlign { x, y }),
            (Some(x), None) => { let x = painter.size_ctx().width(&self.inner).apply_rest().align(x);
                UiRegion::new(x, UiSpan::FULL) }
            (None, Some(y)) => { let y = painter.size_ctx().height(&self.inner).apply_rest().align(y);
                UiRegion::new(UiSpan::FULL, y) }
            (None, None) => UiRegion::FULL,
        };
        painter.widget_within(&self.inner, region);
    }
    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len { ctx.width(&self.inner) }
    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len { ctx.height(&self.inner) }
}
```

```rust
// after
impl Widget for Aligned {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let full = painter.region();
        // Draw once at the full region to learn the child's real size --
        // this placement is provisional and corrected below without a
        // second draw.
        let used = painter.widget_within(&self.inner, full);
        let region = match self.align.tuple() {
            (Some(x), Some(y)) => used.to_uivec2().align(RegionAlign { x, y }).within(&full),
            (Some(x), None) => used.x.apply_rest().align(x).within(&full),
            (None, Some(y)) => used.y.apply_rest().align(y).within(&full),
            (None, None) => full,
        };
        painter.place(&self.inner, region);
        used
    }
}
```

`Painter::widget_within`/`widget`/`widget_at` (`painter.rs:55-76`) change
return type from `()` to `Size`, carrying the child's `draw` result back —
the only signature change needed to let a parent see what its child used.
`Painter::place` moves an already-drawn child when its used area fits the
target box, and redraws it when the target changes its size. `SizeCtx` and
`Painter::size_ctx`/`size`/`len_axis` (`painter.rs:141-150,
180-182`) are deleted — nothing calls `desired_len` any more, so there is
nothing left for `SizeCtx` to answer; `draw_text`/`label`/`px_size`/
`output_size` already exist redundantly on both `SizeCtx` and `Painter`
today (compare `size.rs:71-90` against `painter.rs:152-174`) and this
deletes the `SizeCtx` copies, keeping the `Painter` ones.

### 7. Rejected, and why

- **A flat (non-chained) per-subtree offset table**, Iris's literal
  phrasing — rejected in §2 for breaking under nested independent moves
  (a swiped row inside a scrolling list). Costs nothing extra to avoid: the
  chain is the same mechanism with one more field.
- **Keeping `region_mut` recursion as the only move mechanism** — rejected
  as the steady-state path (O(primitives in subtree), exactly what a
  transcript scroll must not pay every frame) but kept for resize-shaped
  changes (§3) where the content's own region field, not an ancestor
  chain, is what has to change.
- **A general measurement API** — rejected because it walks the same nested
  tree again. The narrow `size_hint(axis)` contract is exact,
  context-free, and optional; it exists only for sizes a widget already
  declares itself.
- **Passing `available` as an explicit parameter to `draw`** (mirroring
  Masonry's `layout(&mut self, ctx, bc: &BoxConstraints) -> Size`, the
  yardstick per AGENTS.md) — rejected as redundant with `Painter::region()`,
  which already carries the same information into every widget that needs
  it; adding a parameter would just be a second route to a value already
  reachable, and would invite the two drifting apart.
- **Eagerly propagating a moved widget's delta into every descendant's own
  offset value** (rather than chaining and resolving in the shader) —
  rejected as O(descendant widgets), which is smaller than O(primitives)
  but still not O(1), and the shader-side chain costs nothing extra to get
  the better bound.

## Density: `Len::dp`, resolved at `apply_rest` time (2026-09-06)

Iris asked for a third length kind beside `abs` (physical pixels) and
`rel`/`rest` (a fraction of the parent) — IRIS_TODO.md's "density-
independent length unit" — after the P0 phone pass found 16px text
drawing at roughly a third size on a real phone. The fix that shipped
first (RUST.md's P0 box) was a global stopgap: divide the whole window
into a "logical" coordinate space (physical ÷ `content_scale`) and let
the shader's NDC mapping stretch it back up onto the real framebuffer.
That fixed the *size* but not the *sharpness* — a glyph rasterised at the
small, pre-stretch size and then stretched onto more physical pixels than
it has texels for is blurry, which is exactly what Iris's next report
said.

**The fix**: `Len` gained a `dp` field, resolved against a `density: f32`
(physical pixels per dp) at the one place a `Len` becomes a `UiScalar`
(`Len::apply_rest`) — `abs + dp * density`. `density` lives on
`UiRenderState` (`set_density`/`density()`) and `Painter` (`density()`),
set once from `DisplayMetrics.density` in `android::view::new_peer`; the
desktop backend has no per-monitor density wired up yet and stays at
`1.0`. Every layout call site that used to call `.apply_rest()`/
`.to_uivec2()` now passes `painter.density()` (nine call sites — `Span`,
`Sized`, `MaxSize`, `Aligned`, `Scroll`, `LazySpan::place`, and
`UiRenderState::place` itself). This also meant the Android
boundary's global logical-space stopgap could come out entirely: window
size, touch coordinates and insets are physical pixels again, matching
`AndroidRenderer`'s own swapchain resolution, with `dp` doing the
per-length work the global divide used to do for everything at once.

**Text is the case that needed more than the `Len` plumbing.** A widget's
`font_size`/`line_height` are plain `f32`, not routed through `Len` at
all (there is no sensible `rel`/`rest` for a font size). `TextBuffer::
shape` now takes `density` directly and multiplies `font_size`/
`line_height` (and any span override) by it before handing them to
parley — so the size that reaches both the line-breaker and the
rasteriser (`TextData::place`, which reads back whatever `shape` set) is
the display's *physical* size, and the glyph atlas holds a bitmap at the
resolution it is actually shown at. `GlyphKey.size` already keys on the
resolved size, so a cache entry is naturally per-physical-size with no
further change. The one caller with no `Painter` to read density from
(`TextEditCtx::layout`, cursor movement and hit-testing) reads a second
copy kept directly on `TextData` (`TextData::density`) instead — an
accepted duplication rather than threading a `Painter` into every input
handler for one field, the same tradeoff `AndroidRenderer::content_scale`
already makes for the Diagnostics page.

**What did not change**: `rel`/`rest` are unaffected (already
resolution-independent, a fraction of the parent). `Span::gap` and
`Padding`'s four sides moved from bare `f32` to `Len` so `dp(...)` works
on them the same as any other size; a bare number is still `abs`,
physical pixels, unchanged.

## Masks with a shape (decided 2026-09-07, built 2026-09-08)

Iris, on the code block's scrolling: "the code block scrolling currently
masks in an inner rectangle. Ideally masks should have a shape
associated with them, rounded rectangle being one of them, and/or
another widget you can select, so that the mask becomes the parent
container with rounded edges. Make sure alpha works properly with it,
eg. on the corners where alpha should be decreased / multiplied."

**What exists.** `Mask` in `shader.wgsl`/`data.rs` is two `UiSpan`s and
a `move_idx`; `fs_main` resolves it and does `color *= 0.0` outside the
rectangle -- a hard cut on a pixel boundary. `Masked` (`widget/mask.rs`)
sets the painter's mask to its own region. Separately, `draw_rounded_rect`
already produces an anti-aliased rounded edge from
`distance_from_rect(pos, center, corner, radius)` with a half-pixel
`smoothstep`, and the border variant multiplies a second coverage in.

**Design** (revised the same day on Iris's two corrections: hit-testing
applies the shape too, and a mask should reference a primitive rather
than carry a copy of its shape).

1. **A mask is a reference to a primitive already drawn, plus how to
   use it.** `Mask { kind, idx, flags, parent }`: the primitive's
   binding (`RECT`, `TEXTURE`, `GLYPH`) and slot, flags (today one:
   *alpha only* -- take the primitive's coverage and ignore its colour,
   which is the default and the only mode until a need for another
   appears), and the enclosing mask's slot for nesting. The fragment
   stage evaluates the referenced primitive *at the masked pixel* --
   for a `Rect`, the same `draw_rounded_rect` coverage from the same
   SDF; for a texture or glyph, the sampled alpha -- and does
   `color.a *= coverage`. Nothing about the shape is copied: a rounded
   container's corner and its children's clipped corner are the same
   primitive's arithmetic, and a texture mask (an alpha image as the
   clip) works with no new shader path.
   What this needs from the data layout: evaluating a primitive at an
   arbitrary pixel means its placement (its spans and `move_idx`, today
   vertex attributes) has to be readable from a storage buffer in the
   fragment stage. If it is not already there, put it there once, for
   every primitive, rather than keeping a second copy for masks -- the
   vertex stage can read the same buffer. Textures: the shader binds one
   image at a time (see `masks_layout`'s comment on why an image's own
   bind group must not name the masks buffer), so a texture mask is
   limited to what the fragment can sample without a bind-group switch:
   the atlas, and the primitive's own bound image when the masked
   primitive is drawn in the same image's batch. Say so at the flag.
2. **Nested masks chain and multiply, like moves.** `parent` walks up
   the chain, bounded like `resolve_move` (`MOVE_CHAIN_LIMIT`'s sibling;
   debug-assert on overflow and print the chain); coverages multiply,
   so a pixel inside two feathered corners is dimmed by both, which is
   what a compositor does and what "alpha should be multiplied" asks.
3. **`.masked()` points the mask at the current widget's own
   primitives.** `Masked` stops describing a region: it records which
   primitive(s) the wrapping widget drew this frame (the painter knows
   -- it just allocated the slots) and sets the mask to reference them.
   So a rounded `Rect` widget's `.masked()` clips its children to
   itself by pointing at the rect it already draws; an image widget's
   `.masked()` clips to its alpha. No radius or shape argument exists to
   fall out of sync. When a widget draws more than one primitive (a
   bordered rect is one primitive; a card with a stripe is two), the
   mask references the *first* and the doc says so; a widget that wants
   another names it.
4. **Hit-testing applies the shape.** A press is inside a masked
   subtree only if the mask's coverage at that point is above one half.
   For a `Rect` that is the same rounded-rect SDF evaluated on the CPU
   -- one function in the shared crate, with the WGSL a transliteration
   of it and a test that compares the two at a grid of points
   (`headless` renders to a buffer and reads back, or the Rust version
   is checked against the values the shader produced once and recorded).
   For a texture, the CPU needs the alpha: keep the alpha channel of an
   image used as a mask readable on the CPU (it was uploaded from CPU
   memory; keeping the alpha plane is a quarter of the image), and read
   it at the point. A masked corner that cannot be tapped and a masked
   corner that is not drawn are then the same corner.

**Rejected.** A stencil buffer (a second pass per mask level and no
anti-aliasing); the scissor rectangle (rectangles only, no alpha);
rendering a masked subtree to an offscreen texture and compositing
(a texture allocation per mask, every frame it scrolls, on the phone).

**Pass conditions.** A headless test draws a rounded container with a
masked child that overhangs all four sides and asserts the child's
coverage at a corner pixel equals the container's own coverage there
(same primitive evaluated, so exactly equal, not approximately); a
nested-mask test asserts the product at a pixel inside both feathers; a
texture-mask test clips a rect to an alpha image and asserts a
transparent texel masks fully; a hit-test asserts a press in a
container's clipped corner misses and one just inside the curve hits,
and that the CPU SDF and the shader agree at a grid of points; a
`run-headless.sh --phone` screenshot of a scrolled code block shows
rounded corners with no square pixels poking out at the top and bottom
of the scrolled content. Record the commands in RUST.md when it lands.

### What was built (2026-09-08), and where it differs

The commands and the screenshot are in docs/RUST.md's queue entry. Four
places the code is narrower than the design above, each deliberate:

- **No `kind` and no `flags` on `Mask`.** It is `{ primitive, parent }`.
  The referenced instance already carries its own `binding`, so a copy
  of it in the mask is a second thing to keep in step; *alpha only* is
  the only mode there is, so there is nothing to select. Both are a
  field away if a second mode appears.
- **A mask's shape must be a rect.** `Painter::set_mask_to` asserts it,
  by name, rather than leaving the shader to read a `rects` entry that
  is not there. A glyph would need a CPU-side alpha plane before the
  hit test could agree with the shader, and a standalone image needs a
  bind-group switch the fragment stage cannot make (`masks_layout`'s own
  comment on why an image's bind group must not name the masks buffer).
  So **the texture-mask pass condition is not met and no texture mask
  exists** — the point of the reference design is that adding one is a
  binding check and a sampled alpha, with no new shader path, and the
  shader's `mask_coverage` already has the branch where it would go.
- **The shape is a primitive of its own, not always a drawn one.** A
  plain `.masked()` writes an undrawn `RectPrimitive` at its region
  (`Drawn::No`, `NOT_DRAWN`) and points the mask at that, so "clip to my
  box" and "clip to that widget's rounded background" are one mechanism
  and square-cornered clipping did not become a special case.
  `.masked_by(shape)` draws `shape` behind the content — in its own
  layer, the way `Stack` puts a background under its content — and
  clips to the first primitive it drew.
- **The CPU/shader agreement is a GPU test**, `iris/tests/mask_sdf.rs`,
  the only test in the workspace that needs an adapter. It lifts
  `distance_from_rect` and `rounded_rect_coverage` out of
  `iris_core::SHAPE_SHADER` *by name* and runs them in a compute pass,
  so the thing under test is the shader itself rather than a copy of it
  that would be edited alongside.

## What a widget's *offered* box may and may not be (2026-09-08)

Two rules that were each true in one place and missing from a sibling,
found together by Iris's 2026-09-08 phone report.

**Padding works in whatever container it is placed in, and is an inset or
an outset depending on how tight that container's region is.** Iris's
own words, 2026-09-08: "padding should work no matter what container a
widget is placed in, and acts as both inset and outset depending on how
tight the parent region is." `Pad` offers its child the region it was
handed, inset on each side, and reports `used + padding` — so given a
generous box it insets the child inside it, and given a box already the
size of the content it reports a larger size and the parent grows. What
this rules out is any container that offers a padded child a box and then
ignores what it reported, and any caller that reshapes its tree to avoid
a `Pad` (which `transcript-ui/src/tool.rs` did until 2026-09-08, at the
cost of a tool group's 4dp inset).

**A widget offered a box it does not fit is drawn again at the box its
own reported size implies, in the same frame.** Not next frame. The
temptation to defer is real — `LazySpan::place` offers a row its *cached*
height precisely so that an unchanged row hits `draw_inner`'s cheap
skip-or-move path, and `Scroll` sizes its child region from last frame's
content length for the same reason. But a `Rect` fills whatever region it
is given (`Size::REST`, and `rect.rs`'s `is_size_independent` doc says
why it must), and `.background(rect(..))` is the ordinary way to style
anything — so a one-frame-stale box is a background drawn at the wrong
size while the text inside it is already right. On screen that is a tool
card that looks closed while its text is there and open while it is not.
A move alone cannot fix a changed size; `Painter::place` redraws in that
case.

The cost is bounded and worth stating, because it is what makes the rule
safe to apply everywhere: the settling draw happens only on the frame a
widget's own size actually changes, which is a frame that was already
redrawing it. `Sized` also requires its final region before retaining its
children: its own reported size may be known exactly while a descendant was
drawn in the provisional box, so moving only the wrapper is insufficient. A
widget whose reported size is a function of the box it was *offered* would
disagree every frame and redraw every frame — which is why `LazySpan` requires
content-sized rows, and has since long before this.