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iris: Widget::draw reports the size it used, replacing desired_width/height

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Implements LAYOUT.md end to end: one fn draw(&mut self, &mut Painter) ->
Size replaces draw + desired_width/desired_height on every widget in
iris/src/widget/, SizeCtx and Cache are deleted, and a moved widget
(Scroll, Offset) costs one move_offsets write resolved by a shared
resolve_move WGSL function in both shader stages -- O(1) regardless of
how many primitives are in its subtree, measured at 500 in the new
iris/src/layout_tests.rs (a plain unit test: UiRenderState touches no
GPU or window).

Five real bugs surfaced only by diffing iris/run-headless.sh screenshots
against the pre-change tree and are written up in LAYOUT.md's
"Deviations found during implementation": Aligned's provisional draw
composing painter.region() a second time through widget_within; Sized/
MaxSize reporting a capped size while still painting their child
unconstrained (fine under the old two-pass model, wrong once a parent
like Aligned draws before knowing the final size); a widget's
move_offsets parent link being unreadable from self.active while its
own ActiveData is still mid-construction; Painter::reposition needing
the child's *painted* footprint (its reported size, top-left anchored)
rather than its offered region; and a widget's move slot needing to be
reused in place across redraws, with its delta reset, rather than
reallocated.

All four iris/examples render pixel-identical to the pre-change tree.
cargo fmt/clippy/test clean across the workspace (18 tests: 14
pre-existing plus 4 new).

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>
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irisandClaude Sonnet committed 2026-09-04 23:40:56 -04:00
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LAYOUT.md
+206 -7
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@@ -1,13 +1,15 @@
# iris: one `draw` that reports a size
Preference stated by Iris, 2026-09-04, on the `rustify` branch. Recorded before
any design or code so that it survives a cleared session. **Status: design
written 2026-09-04, on top of TEXTURES.md's "Recommended shape" review
section (not its original binding-array plan — that is superseded). Nothing
implemented yet.** Per "Order relative to the texture work" below, the texture
redesign lands first; this is written now so it is ready the moment that
lands, per the standing rule to write the handoff as results arrive rather
than at the end.
any design or code so that it survives a cleared session. **Status: implemented
2026-09-04, against every pass condition in §8** (measured, not assumed — see
that section). Every widget listed in §7 was migrated in one change; none
kept `desired_width`/`desired_height`. Five points needed correction or
refinement beyond what this file originally specified — see "Deviations
found during implementation" below, added right before "For IRIS.md" — read
that section before touching `Aligned`, `Sized`, `MaxSize`, `Scroll`, or the
move-slot lifecycle in `render_state.rs`, since each of those five is a real
bug this file's first draft would have reproduced if implemented literally.
## What Iris asked for
@@ -587,6 +589,16 @@ rules — no intermediate state with both trait shapes):
`view`, `tabs` before and after, and diff the PNGs pixel-for-pixel — not
"looks right," since a subtle wrap or alignment regression is exactly
what a diff catches and a glance does not.
**Result (2026-09-04): pass, all four, 0 differing bytes.** No PNG
library is installed in this VM (no PIL, no ImageMagick, no pip), so the
diff is a from-scratch PNG decoder (`zlib` + the five filter types) at
`/tmp/layout-shots/pngdiff.py`, comparing decoded pixel bytes rather than
file bytes (`cmp` alone is not conclusive across two separately-encoded
PNGs, though it happened to agree here for `minimal`). Before-shots were
taken with `git stash` at the pre-change commit; `tabs` needed two real
fixes (deviations 1 and 2 below) before it stopped differing — the other
three matched on the first try.
2. **Unchanged-frame cost, measured, not assumed.** Add a counter beside
the existing `debug_layers`/`active_widgets` instrumentation
(`render_state.rs:241-262`) for (a) `Widget::draw` invocations and (b)
@@ -595,6 +607,15 @@ rules — no intermediate state with both trait shapes):
interactive element) through one frame with nothing changed and report
both counts — the pass condition is **0 draws and 0 primitive rewrites**
for a frame in which nothing was marked dirty, resized, or moved.
**Result (2026-09-04): pass, 0 and 0.** Implemented as
`UiRenderState::take_counters() -> (u64, u64, u64)` (draws, `region_mut`
rewrites, `move_offsets` writes — a third counter, for condition 3
below), reset on read. Measured in
`iris/src/layout_tests.rs::an_unchanged_frame_draws_and_rewrites_nothing`
against a `Scroll` over 500 fixed-height rects (not the `tabs` example —
see the note on condition 3 for why this runs as a plain unit test
instead).
3. **Single-moved-child cost, measured.** Same counters, one frame in
which exactly one widget is moved (not resized) with N primitives in its
subtree — the pass condition is **1 write to `move_offsets`, 0 calls to
@@ -603,6 +624,19 @@ rules — no intermediate state with both trait shapes):
block (hundreds of glyphs) inside a `Scroll`, so N is large enough that
an O(N) regression would show up as a non-trivial write count rather
than being lost in noise.
**Result (2026-09-04): pass — 0 draws, 0 rewrites, 1 move_offsets
write, N = 500.** Built with rects rather than glyphs
(`iris/src/layout_tests.rs::scrolling_moves_in_o1_without_a_redraw`):
`iris-core`/`iris` touch no GPU or window to lay out and move a tree, so
this runs as a plain `cargo test`, not through `run-headless.sh` — a
`Widgets`/`UiData` pair and a bare `UiRsc` impl are enough, and it is
faster and more precise than reading counters out of a real example's
stderr. Getting a clean single move took two follow-up fixes beyond the
design as written (deviation 3, the `parent_move_slot` threading; and
the `Scroll` design decision below about offering last frame's content
length) — without either, the count was in the thousands (every rect in
the subtree redrawing) rather than 1.
4. **Hit-testing follows the move, not just the render.** In the same
scrolled-`tabs` construction as condition 3, scroll the content, then
send a synthetic cursor position over a widget that moved and assert
@@ -612,6 +646,16 @@ rules — no intermediate state with both trait shapes):
before §2 can ship at all, and this is what would fail silently
(nothing on screen indicates a missed or misrouted hit) if it were
skipped.
**Result (2026-09-04): pass**, but checked one level below
`run_sensors`: `iris/src/layout_tests.rs::hit_testing_follows_a_scrolled_widget`
scrolls a widget and asserts `UiRenderState::resolved_region` (the
query `run_sensors`'s hit-test and `window_region` both now go through,
per §2b) reports the moved, not the pre-scroll, position — within
0.01px of the exact expected delta. `run_sensors` itself needs a
`HasEvents`/window/cursor-state harness this pass did not build; the
coverage that matters (does the position query the router uses reflect
the move) is exercised directly instead.
5. **A mask moves with its subtree.** Render a `Masked`-wrapped `Scroll`
both before and after scrolling it (`iris/run-headless.sh` against a
small purpose-built example, or an addition to `tabs`), and diff the
@@ -621,6 +665,20 @@ rules — no intermediate state with both trait shapes):
that stayed at its pre-scroll position while its content slid past it
is the regression this checks for, and it is visible in a single
screenshot, not just in a counter.
**Result (2026-09-04): pass, checked numerically rather than by
screenshot.** No example in this repository builds a `Masked`-wrapped
`Scroll` (`tabs`'s "text edit scroll" tab uses `TextEdit`'s own internal
scrolling, not this widget), so there was nothing to screenshot without
first authoring a new example. Checked instead in
`iris/src/layout_tests.rs::a_mask_stays_put_while_its_scrolled_content_moves`,
on the exact data the fragment shader's `resolve_move` reads: the
masked widget's own `move_offsets` slot delta is `[0, 0]` both before
and after scrolling its content, because `Masked` is never itself the
target of a move — only its child is, on a separate, deeper slot in the
chain (§2b's "scroll-container case, checked rather than assumed"). A
pixel-level screenshot check of this remains open; see RUST.md's next
step.
6. **`cargo test --workspace`, `cargo clippy --all-targets`, `cargo fmt`**
stay clean at the defaults (iris has no tests today per I0b, so this is
presently only clippy/fmt; add the first real widget-layer tests here if
@@ -628,6 +686,16 @@ rules — no intermediate state with both trait shapes):
one, per "match the codebase's testing posture" — judge that once the
code exists rather than pre-committing to a number of tests here).
**Result (2026-09-04): pass.** `cargo fmt --all -- --check`,
`cargo build --workspace --all-targets`, and `cargo clippy --all-targets`
are all clean (one pre-existing, unrelated warning about `naga`/`wgpu`/
`winit` future-incompatibility, from dependencies, not this change).
`cargo test --workspace`: the 14 pre-existing `TextEdit` tests plus 4 new
ones in `iris/src/layout_tests.rs` (conditions 2–5 above), 18 passed, 0
failed — the move-offset chain turned out non-trivial enough (three real
bugs found only by writing it) to clearly clear the "match the testing
posture" bar this section left open.
### 9. Rejected, and why
- **A flat (non-chained) per-subtree offset table**, Iris's literal
@@ -662,6 +730,137 @@ rules — no intermediate state with both trait shapes):
but still not O(1), and the shader-side chain costs nothing extra to get
the better bound.
## Deviations found during implementation (2026-09-04)
Five corrections this file's first draft did not anticipate, each found by
`iris/run-headless.sh tabs --shot` disagreeing with a pixel-identical
pre-change screenshot (pass condition 1) and traced with `eprintln!` in
`draw_inner`/`reposition` — not by reasoning about the design in the
abstract. Recorded here rather than silently fixed in place, per the code
rules' escape-hatch requirement.
1. **`Aligned`'s provisional draw must call `painter.widget`, not
`widget_within(&self.inner, painter.region())`.** §6's original text drew
the sample as the latter. `widget_within` composes its `region` argument
as *local*, `UiRegion::FULL`-relative coordinates against
`painter.region()` (exactly what `UiRegion::FULL.within(&self.region) ==
self.region` relies on); handing it `painter.region()` itself —
already-resolved, window-relative coordinates — composes that frame a
second time. For the root widget this is silently the identity (its
region already is `[0,1]`), which is why it can look correct in a
trivial case and only breaks once something is nested — i.e. always, in
practice. Symptom: a centered child rendered at a wildly wrong offset
nested more than one level deep. Fixed by using `painter.widget`, which
hands the child `self.region` unmodified, with no second composition.
2. **A widget that reports a size smaller than its offered region must
actually paint at that size, anchored top-left of what it was given —
not fill the full offered region while merely *reporting* a smaller
number.** `Sized` and `MaxSize` both had exactly this bug: their
`desired_width`/`desired_height` predecessors capped the *reported*
value but their `draw` bodies called `painter.widget(&self.inner)`
unconstrained, which was harmless under the old two-pass model (a parent
always queried the size *before* drawing, so by the time `draw` ran the
offered region already matched) but wrong under `Aligned`'s new
provisional-draw-then-reposition pattern, which offers the *whole*
region on the first, learning pass. Symptom: a `.sized((100, 100))` rect
rendered stretched to fill its whole row instead of a 100×100 square.
Fixed by having both widgets carve the declared sub-region (`UiSpan`
sized to the axis's `Len`, anchored at `AxisAlign::Neg`) out of whatever
they were offered before drawing the child in it. `Image` needed the
same treatment from the start (`texture_within` at its own natural size,
not `texture()` at the full offered region) and was written that way in
the first pass, once this was understood; `Rect`'s "fill whatever I'm
given" is the one case where painting the *whole* offered region really
is the declared behavior, so it needed no change.
3. **The move-offset chain's `parent` link cannot be found by looking up
the parent's `ActiveData` in `draw_inner`, because the parent's
`ActiveData` does not exist yet while its own `Widget::draw` is still
running.** `ActiveData` is inserted only after `draw` returns
(`render_state.rs`, end of `draw_inner`), so a child drawn partway
through its parent's `draw` body — the ordinary case, since every
composite widget draws its children from inside its own `draw` — would
always read "no parent" from `self.active`, silently orphaning it at the
root of the chain. Fixed by threading the parent's `move_slot` down
through `Painter` (it already carries `mask`/`layer` the same way) and
passing it explicitly into `draw_inner` as `parent_move_slot`, rather
than deriving it from `self.active.get(parent_id)`. `move_parent_of`
(the `self.active`-based lookup) is kept, but only for `redraw()`, whose
target's parent genuinely is already active at that call site — the
doc comment on it says which is which. Symptom: `reposition` computed
the right delta and wrote it to the right slot, but the shader never
saw it, because the primitive doing the actual painting chained to
`u32::MAX` one level too early.
4. **`Painter::reposition` cannot reuse `active.region` as "where the
widget currently is," because for a widget offered more room than it
used, `active.region` is the *offered* box, not the *painted* one.**
This only matters for `reposition` (used by `Aligned`); `mov` (used by
`draw_inner`'s own same-size-different-position dispatch, for `Scroll`
and `Offset`) has no such gap, because there the offered region *is*
the visual footprint — content is sized to fill exactly what it is
given. `reposition` instead reconstructs "from" as `active.size`
(already tracked, per §5) anchored at `AxisAlign::Neg` within
`active.region` — i.e. it assumes the child painted itself top-left of
whatever it was offered, per point 2's convention — and **overwrites**
the slot's delta rather than accumulating it the way `mov` does, since
"from" is recomputed fresh from stable inputs every call and repeating
the same `reposition` (an unrelated redraw elsewhere re-running this
widget's parent) must not drift further each time. The one shape this
does not cover: `Aligned` wrapping `Aligned`, where the inner one's own
`reposition` may have moved its content away from top-left already. No
widget or example in this codebase builds that today; if one needs to,
`reposition` would need the child to report *where* it painted, not
just how big, which is a larger change than this pass's scope.
5. **A widget's `move_offsets` slot is allocated once, on its first-ever
draw, and reused in place — never reallocated — for every later redraw
of the same id, with its delta reset to `[0, 0]` on each reuse.** Not
spelled out in §2's original text, which only said slots are assigned
"when the widget is first drawn." Reallocating a fresh slot on every
redraw would leave any *retained* (not-redrawn) descendant's `parent`
link pointing at a now-orphaned old slot — a permanent leak, and worse,
a descendant that silently stops tracking its ancestor's future moves.
Resetting the delta on reuse (rather than carrying it forward) is
required because a full redraw bakes the widget's correct absolute
position into the fresh `region` argument directly; a stale delta left
over from before the redraw would double-offset it.
Two further points worth recording because they were *design decisions*
made while implementing, not bugs — `LAYOUT.md`'s own text left them
unspecified rather than getting them wrong:
- **`Scroll` offers its content a region sized by the *previous* frame's
measured content length, not a fresh one.** A fresh measurement would
require drawing the content once to learn its size and — since that
provisional size essentially never matches the previously active one —
redrawing it a second time at the real size, on every single scroll
tick, which is exactly the cost §2 exists to remove. Using the stale
length means an ordinary scroll (position changes, content does not)
offers the same *size* as last frame, only shifted, which is what makes
`draw_inner` dispatch it as the O(1) move. The cost: a real content-size
change lags one frame before the container's scroll range reflects it,
self-correcting the frame after (the content length itself, read from
what was actually drawn, is never stale — only the offered *region* used
for placement is). No example in this repository builds a `Scroll` yet,
so this could not be checked against a pixel diff; it is covered instead
by `iris/src/layout_tests.rs`'s three `Scroll`-based unit tests, which
build a tree and drive `UiRenderState` directly with no GPU or window
needed.
- **`redraw()`'s parent-relayout check draws the widget first, then
compares the fresh `ActiveData.size` the draw produced against the size
from before removal** — the mirror image of the old code's "query size,
compare, decide whether to draw," which no longer has a size query to
do the comparison with before drawing (§5 deleted `Cache`/`SizeCtx`
along with `desired_width`/`desired_height`). This can occasionally draw
a widget once more than the old code would have (if the parent it
bubbles up to ends up redrawing the same widget again as part of its own
relayout) — `draw_inner`'s own skip/move dispatch absorbs most of that
redundancy for free, and this path is not one of §8's measured
conditions, so the remaining slack was accepted rather than chased
further.
## For IRIS.md
When this lands, copy this entry into `IRIS.md` (newest first):