ai-app/docs/IRIS_EXTRACTION_HANDOFF.md

# Iris extraction handoff

Operational handoff for pulling Iris out of ai-app into a standalone framework.
Not a decisions log; delete it when the extraction is done.

## Where things stand

Canonical `main` is **`ca2b4b2`** (#17, the headless rig). Sixteen slices are
in.

**#18 `split/18-position-chain`** is open and finished apart from one decision:
worktree `/home/bob/repos/iris-pr18`, head `3f7cd82`, twenty-four commits,
workspace tests passing, fmt and clippy clean. It is LAYOUT.md §2's position
chain, generalised to boxes. The last six are a performance pass that changes
no layout: the two halves of the text fix (`e5f8b6b`, `2525637`), three
retained-layout details that claimed something they did not do (`f1a47e9`),
the many-widgets-dirty cases (`77bb75e`), what the settle order is holding up
(`bf94380`), and carrying a widget's depth rather than walking for it
(`3f7cd82`). Every one of them keeps the five reference renders and the resize
render byte-identical, and the hundred-seed sweep passes. `/home/bob/repos/ai-app-2` is on `rustify`, worktree clean.

**The one thing waiting on the owner.** `tabs` at 1920x1200 is no longer
byte-identical to `upstream/main`: 1,283 pixels of 2.3M (0.06%), two
one-pixel-wide panel edges shifted by a pixel, at x=1056 and x=1337. `view`,
`minimal` and `text` are identical. Composing a position through the chain in
the shader associates the arithmetic differently from collapsing it on the CPU,
so a value that used to land exactly on an integer falls the other side of the
shader's `floor`. The CPU and the GPU still agree with each other -- both walk
the chain bottom-up -- so hit testing matches what is drawn; what changed is
only the comparison against the old code. Matching it exactly means composing
root-down in the shader, which needs the chain collected into an array first.
Byte-identical against `upstream/main` has been the bar for every slice, so
this is hers to accept or to spend a commit on.

Check for a review before starting anything, and read the newest `submitted_at`
rather than the first result:

```sh
TOKEN=$(cat ~/.config/gitea/token)
N=18
curl -s -H "Authorization: token $TOKEN" \
  https://git.arirex.me/api/v1/repos/iris/iris/pulls/$N/reviews
curl -s -H "Authorization: token $TOKEN" \
  https://git.arirex.me/api/v1/repos/iris/iris/pulls/$N/reviews/<id>/comments
curl -s -H "Authorization: token $TOKEN" \
  https://git.arirex.me/api/v1/repos/iris/iris/issues/$N/comments
```

My replies are ordinary issue comments on the same PR and say what each change
was for.

## How a position resolves now

Invariants, not history. Everything in `core/src/ui` rests on them.

- **A slot holds a box, in the coordinates of the slot it names.** A primitive
  instance and a mask each name one, and `prelude.wgsl` composes the chain with
  `within`. A translation is the special case where the box has its parent's
  relative extent. The identity is `UiRegion::FULL`, **not zero** -- a zeroed
  entry is a box of no extent and collapses its subtree to a point, which
  `MoveOffset`'s comment says beside the `Zeroable` that `Pod` requires.
- **A slot has to carry a whole box** rather than a scale and an offset: a
  pixel-space affine map scales everything under it, including a child that
  must keep its pixel length, and the `rel`/`abs` pair is exactly what
  distinguishes the two.
- **Slots are opt-in.** `Painter::place` draws a child whose box its parent
  decides and may decide again, and that child gets a slot; `widget` and
  `widget_within` do not, and share the nearest ancestor's. `Span`, `Aligned`
  and `Scroll` place. This is what keeps the chain 2-4 deep rather than full
  tree depth, which is the difference between free and +42.6%.
- **A widget's region is held in the coordinates of the slot it draws in**, so
  a placed widget draws against `UiRegion::FULL` and its box lives in its slot.
  `ActiveData::region` is the box it was *offered*, in its parent's slot
  coordinates, and `ActiveData::parent_move` is the slot that is in;
  `window_region` composes the one through the other, which is the walk the
  shader does.
- **Nothing inverts a lerp.** `UiRegion::stretch`, `stretchable` and
  `UiScalar::stretch` are gone. A box that changed length is written to its
  slot and the descendants recompose against it, which also covers the case the
  old guard refused outright: a fixed length has no fraction to recover, so a
  40-tall row could not be stretched on its other axis at all.
- **Reuse is decided on the box a widget drew against, in pixels**
  (`ActiveData::px`). A region is a fraction of a slot's box, so an unchanged
  region is *not* an unchanged box -- a child drawn at `FULL` of a slot that has
  since halved compares equal to itself. This is the check everything else
  rests on; do not weaken it back to comparing regions.
- **A size the parent learnt by drawing the child is an answer for that box
  only.** `redraws_under` redraws a child whose size the widget read unless it
  declares an exact `size_hint` for the changed axis -- the one case the parent
  did not have to draw it to find out. The cost is that a size-reading
  container gives up its reuse when its box changes length, which is every
  span, so `OnResize::Scale` earns its keep on moves and on subtrees whose
  sizes nobody read rather than on every stretch.
- **`redraws_under` is a question asked before reusing, never a marking.**
  Marking descendants for redraw instead does not terminate: the mark escalates
  to that descendant's size reader, which re-places the child, which marks it
  again. Asking first and giving up the whole reuse adds no marks and stops.
- **The walk stops where a length did not change.** A part of a box with no
  relative extent on an axis is a fixed length held as offsets from that box's
  start, and composing anything into it leaves no relative extent either -- so a
  widget whose own box did not change length has no descendant whose box did.
  An 80-wide child of a widened row is never asked.
- **`Span`, `Pad`, `Stack`, `Offset`, `Aligned`, `SetSize` and `LayerOffset`
  say `Scale`**; each places in fractions and offsets of its own box and none
  reads its pixel length. `Scroll` and `MaxSize` read pixels and stay `Redraw`,
  which is the general rule: `Scale` on an axis unless the draw reads the pixel
  length of its box on that axis. The default stays `Redraw`.
- **`OnResize::Scale` keeps its name.** The owner rejected `Stretch` on
  2026-09-14: stretch has an opposite and scale does not, and the answer is per
  axis, so the axis is already established where it is read.

## Measured, so the next attempt is compared rather than argued

| rig | what it says |
| --- | --- |
| `tests/chain_cost.rs` | GPU pass time by chain depth at 200k instances. Translate slots: free to depth 8 (+5%), then ~3 us per level, +42.6% at 16 and +221% at 64. Each step is a storage load addressed by the previous one, so it is the chaining that costs, not the arithmetic at a level. A box slot (36 bytes) against a translate slot on the same binary: +0.6% at depth 1, +0.5% at 2, +0.8% at 4, then +9.6% at 8 and +32.2% at 64 -- free where opt-in slots put it, and dear only where the chain already was. |
| `tests/replace_cost.rs` | Instructions per frame re-placing 200 rows: 1.98M writing each row's slot, 2.38M rewriting its regions, 7.13M redrawing it. A load for `perf`, not a check. Five primitives per row; the regime that decides whether the chain is worth it is a transcript row of a few hundred glyphs, **so re-run it with 200 characters of text per row before concluding anything from it**. |
| `tests/draw_cost.rs` | What recording a frame costs on the CPU by layer count. Dispatch per list is 6 instructions, 0.1% of a frame at 256 and at 1024 layers. |

A chain is irrelevant at an example's couple of hundred primitives; a
transcript's glyphs are tens of thousands, which is the regime `chain_cost`
measures.

### How much of that work is necessary

Measured 2026-09-14 on a random tree at seed 1, depth 7 -- 1061 widgets, 839
of them drawn, 10,872 primitive instances -- against the same rig at `43ce8c7`
(the last commit before #16 sized a widget by drawing it) and at `f942385`
(#16 itself). The rig is a `Harness` load counting widget draws, text shapes
and instance writes per frame, plus the GPU pass through timestamp queries;
it lives in the scratch worktrees `/home/bob/repos/iris-size-{old,new}` and is
not in the repository, because the counters it reads are patched into
`iris-core`.

| per frame | before #16 | #16 | #18 head |
| --- | --- | --- | --- |
| cold layout | 19.3 ms, 839 draws | 30.9 ms, 3317 | 23.6 ms, 2755 |
| repaint one leaf | 0.000 ms, 1 draw | 8.5 ms, 1683 | 6.5 ms, 1313 |
| scroll one scroller | 0.001 ms, 1 draw | 8.9 ms, 1683 | 6.5 ms, 1313 |
| resize the output | 3.2 ms, 839 draws | 31.8 ms, 6940 | 25.2 ms, 5512 |
| GPU pass | 0.129 ms | 0.114 ms | 0.115 ms |

**The GPU is not the subject.** The pass is a tenth of a millisecond at every
revision and every phase; all of this is the CPU laying out.

`a640c6c` and `84f589e` remove most of the CPU work without weakening the
retained-layout rules. Against the same seed-1/depth-7 load, widget draws are
now 2,117 cold, 1 for a leaf repaint, 11 for a scroll, and 3,139 for a resize
(from 2,755, 1,313, 1,313, and 5,512 respectively). Workspace tests pass; the
five reference renders, resize render, and image-tab replay are byte-identical
to the prior #18 head.

Those draw counts explain mechanism, not total layout cost. Before #16,
measurement was a separate operation: the cold and resize rows each made 839
draws plus 489 size queries, 314 of which hit the size cache. Current sizing is
a draw, so its draw count includes the provisional work and cannot be compared
directly with the old draw count.

### Against the old code, on a tree both revisions build

**The random generator is not a fixed load, so it cannot carry an old-versus-new
comparison.** Padding, scrolling and alignment grew into it during this work, so
`seed 1, depth 7` is 144 widgets today and was the 1061-widget tree the rows
above were measured on. Earlier statements here that the retained layout was
"about 1.6x the old 3.14 ms resize" compared a 260-widget depth-8 tree against
that 1061-widget depth-7 one, and are withdrawn.

`tests/revision_cost.rs` is the load that does carry it: one hand-written tree
-- 40 rows of `Dir::RIGHT` span holding a 40px rect and a `Dir::DOWN` span of
one wrapping and one short non-wrapping text, each its own random words -- in
source that compiles unchanged on `43ce8c7` and on #18. Drop it into an old
worktree and run it there; take the number from `perf stat -e instructions:u`
on the test binary rather than the clock. It also prints where the layout put
the paragraphs, which is the other half of the comparison, and `text_memory`
in the same file reports what the tree holds.

**The old code is faster because it skips the question, not because it answers
it more cheaply.** Two shortcuts pay for the 2.2x, and both are wrong:

- `SizeCtx::len_inner` keys its memo on the widget id alone. It stores the
  constraint box the answer was computed under and never compares it, so the
  first asker's box decides the answer for every later one -- 314 of those 489
  hits per frame. The frame-local answer cache rejected on seed 98 at least
  compared the box; this does not.
- `Span::len_sum` measures every child against the container's whole axis
  rather than the part left for it, which its own comment records as a
  deliberate choice ("tempting to subtract the abs & rel from the ctx outer,
  but that would create inconsistent sizing"). In the fixture above that shapes
  a paragraph at 900 rather than the 860 left beside the rect, and it draws
  29 px past the edge of the output.

So the amplification #18 pays is the cost of asking in the right box. That does
not excuse the size of it, and most of it turns out not to be layout at all.

`480f0bc` retains that instrumentation behind the `layout-diagnostics` Cargo
feature; none of it is compiled into a normal Iris build. The ignored
`tests/layout_diagnostics.rs` rig selects `cold`, `repaint`, `size`, `scroll`,
or `resize` with `IRIS_PHASE`, plus seed, depth and frame count. Run it with the
feature for explanatory counters and inclusive phase timers, and without the
feature under `perf` for unperturbed CPU totals:

```sh
IRIS_PHASE=resize IRIS_DEPTH=8 IRIS_FRAMES=100 \
  cargo test --release --features layout-diagnostics \
  --test layout_diagnostics -- --ignored --nocapture

IRIS_PHASE=resize IRIS_DEPTH=8 IRIS_FRAMES=1000 \
  perf stat -e cycles:u,instructions:u cargo test --release \
  --test layout_diagnostics -- --ignored --nocapture
```

`82fa6c1` adds targeted tracing to the same feature. Call
`layout_diagnostics::trace_widget(id)` before a frame; `take().traces()` then
returns the selected widgets' ordered draw requests and pixel boxes, reuse
outcomes, placements, hints, size reads and reported sizes, and text widths.
The selection is a set, survives `take()`, and is removed with
`untrace_widget` or `clear_traced_widgets`. This replaces temporary
text-specific logging without imposing anything on normal builds.

The first depth-8 resize run made the amplification concrete. A 260-widget
tree has 215 active widgets, but a resize averaged 1,395 widget draws, 913
placement calls, 1,313 reads of drawn sizes and 34,844 primitive writes. Only
12 distinct text widgets rendered, yet they rendered and reshaped 282 times
per frame with no shape-cache hits. Text rendering accounted for 11.6 of 13.2
ms, including 9.9 ms shaping and 1.7 ms placing glyphs. The hottest two text
widgets each drew 96 times below nested `Span`, `Aligned`, `Scroll`, `Pad`, and
`SetSize` ancestors.

Tracing one of them showed that 96 was two multipliers, not 96 necessary
layout iterations. One traversal of the nested positioning chain shaped it 32
times at 12 widths; three resize-dependent descendants then caused that whole
traversal to run three times through the same highest size reader. Resize
marking had queued each pixel reader independently, and each leaf discovered
and redrew the same reader path in turn.

`82fa6c1` coalesces that frontier by marking every resize-dependent leaf and
its size-reader chain first, then settling the shallowest shared reader once.
Ordinary content and size changes remain deepest-first. On the same depth-8
load, an actual resize now averages about 694 widget draws, 435 placement
calls, 649 drawn-size reads, 15,343 primitive writes, and 116 text shapes; the
hottest texts draw 32 times. Instrumented layout is about 5.5 ms instead of
13.2 ms. An uninstrumented 1,000-frame resize run measured a 5.61 ms median,
60.79B instructions total (60.8M/frame), and 23.85B cycles. This is about 58%
fewer instructions than the roughly 138M/frame #18 path before coalescing.
The remaining multiplier is within one constraint traversal. Re-sending the
existing output size also no longer starts a resize.

`b1b3eca` records pixel reads and retained size inputs per axis. `Text` says it
reads only its offered width, while `Scroll` says it reads only its scrolling
axis. `Span`, `Aligned`, and `Scroll` ask for an exact hint or a retained child
length valid under the box they are about to offer, and draw to measure only
when neither exists. On the same depth-8 load, resize now averages 449 widget
draws, 296 placement calls, 485 drawn-size reads, 12,164 primitive writes, and
107 text shapes. Cached uninstrumented 1,000-frame runs measured 4.95–5.24 ms
medians, 55.4B instructions total (55.4M/frame), and 21.5–21.6B cycles: about
9% fewer instructions than `82fa6c1`.

Pixel comparisons now use a 0.05 physical-pixel tolerance. The comparison is
against the last actual layout, not the preceding resize event or retained
move, so repeated subpixel changes accumulate and eventually redraw. The
generated cold-layout oracle uses the same visual tolerance per coordinate.

The generated tree now includes `Aligned` with every meaningful per-axis
alignment. That exposed two retained-layout ordering bugs which `84f589e`
fixes. The regular cold-layout equivalence suite and the ignored 100-seed
sweep pass. The latter previously stopped at seed 60 on `140.0` versus
`139.99996948242188`: exactly two `f32` ULPs from an equivalent composition
order, not a visible layout difference. The oracle now keeps draw presence
exact and allows at most 0.05 physical pixels per coordinate. Seed 98 is also
part of the ordinary suite because it catches stale retained size beneath an
aligned size-changing chain.

The branches are invariants rather than widget exceptions:

- A dirty widget draws locally first only while its retained box has the same
  pixel size. If its returned `Size` is unchanged, no size reader can observe
  the repaint and no ancestor draws. If the size changed, the dependent path
  lays out. A changed pixel box takes the conservative path first, which is the
  condition the discarded `/tmp/escalate.patch` missed on resize.
- Dirty widgets settle deepest-first. A changed size queues only its immediate
  reader; propagation stops as soon as a reader's own answer stays unchanged.
  Output resize is the distinct invariant: all pixel-dependent leaves and
  their reader chains are marked together, then shared reader roots settle
  shallowest-first under the new output. This both chooses the parent box
  before drawing its descendant and coalesces overlapping resize paths.
- A span measures an unknown child in the part of its axis still available,
  rather than giving every child the whole container and immediately taking
  most of it away. This is the archive's faster shape, recreated on the current
  types; it often makes the measurement box the final box without caching an
  answer under different constraints.

A proposed `Scroll` shortcut that reused its direct child's retained size was
discarded. “Direct child is clean” is not strong enough while a dirty
descendant's structural change is still propagating; seed 52 demonstrated the
stale-size failure. Re-measuring the scroll subtree costs 11 draws rather than
1, but remains two orders of magnitude below the old 1,313 and follows the
actual dependency invariant.

A frame-local cache of several size answers under different constraint boxes
was also discarded. It reached about 3.5 ms in the instrumented resize rig,
but seed 98 showed that a valid answer is not enough: the child's one active
drawing may still realize another constraint, and a parent can then lay out
from an answer its final child placement does not realize. A correct version
needs an answer-plus-realization or verification protocol; adding dirtiness
conditions to the cache does not fix that invariant.

Do not restore the archive's old `DrawMode::Measure`. It skipped primitive and
retained-state writes while still walking widgets and shaping text, and once
improved a streamed-frame benchmark from 1.39 to 1.22 ms p50. Retained
placement later replaced it because a provisional draw is usually already
usable in its final box; measure-only mode would discard that useful output
and force another real traversal. The owner confirmed on 2026-09-14 that
direct placement is the intended path.

### What a resize frame is actually spent on

`perf record` on #18's head, resize phase, seed 1 depth 8: about 63% of the
frame is text -- shaping in parley, harfrust and icu\_segmenter, then
`TextData::place` at 10.8% on its own -- and `draw_inner` is 0.9%. **The
retained-layout machinery is not what the frame costs.** A container that
measures by drawing makes a text draw 32 times at about a dozen distinct
widths; every one of those re-runs the shaper, because the caches in front of
it (`TextView::width`/`tex` and `TextBuffer::layout_key`) each hold exactly one
entry and a trial width alternating with a final width evicts the answer that
is about to be asked for again.

`e5f8b6b` and `2525637` are the two halves of the answer.

**A new width is a line break, not a shaping.** Only the breaking depends on
the width -- the font selection, the unicode analysis and harfrust under it are
a function of the text and the attrs, and parley re-breaks them in place, which
is what its own editor does on every resize. On the depth-8 tree that is 107
breaks at 0.119 ms where the shapings they replace were 4.0 ms. This is the
half that holds however far the width moves.

**A bounded store on `TextData` holds what re-breaking still cannot avoid:**
the glyphs, placed per width, keyed by the text, the attrs and the width, with
`TextBuffer` holding the pair it is drawn as -- which is where `TextView::tex`
was, so there is one place to invalidate rather than two. Bounding the store
rather than each buffer is the point: a per-buffer cache of eight cost +79% on
a tree of 4,000 texts, and lazy eviction cannot fix that, because the texts
holding the memory are exactly the ones a retained layout has stopped drawing.

Instructions per frame over 500 resize frames of `tests/revision_cost.rs`, for
widths that alternate -- the friendly case for anything that remembers an
answer -- and widths that never repeat, which is a drag:

| | alternating | never repeating |
| --- | --- | --- |
| before #16 (`43ce8c7`) | 56.1M | 56.2M |
| #18 head (`b1b3eca`) | 124.2M | 123.6M |
| the store alone | 17.7M | 45.9M |
| re-breaking alone | 32.9M | 32.8M |
| both | 20.6M | 24.2M |

Neither alone is good at both, which is the reason for having both. The frame
times that go with the last row are 1.88 ms median and 2.54 ms worst
alternating, 2.46 and 3.24 never repeating, against 4.99 and 6.47 before #16 --
so the worst frame more than halves, and the two gestures are within a
millisecond of each other rather than a factor of two apart.

Memory on the same rig at 2,000 rows: 108.6 MB on #18's head, 111.6 MB with
both, and **132.0 MB before #16**, which grows 23 MB over the resizes where
neither of the others grows at all.

Widget draws do not move at all -- 449 either way -- so none of this touches a
retained-layout rule. That matters for more than purity: **text is the load
that makes a redundant draw expensive, and so the thing that shows when the
layout is drawing more than it needs to.** The counters keep saying so -- a
text render is counted per ask rather than per shaping, and breaks and glyph
placements are counted separately -- so neither half can hide the amplification
it is paying for.

This is not the frame-local answer cache seed 98 rejected. That one let a
parent lay out from a size the child's one drawing might not realize; this
holds a *drawing* keyed on what produced it, and the widget still draws. The
five reference renders and the resize render are byte-identical, and the
sweep passes.

**Why picking any dirty widget does not work, found 2026-09-14.** `try_reuse`
asks whether the widget in front of it is dirty and, if not, hands its parent
the size it last reported. It does not ask whether a dirty widget sits under it
through the size dependencies -- which is the check `retained_size` makes, on
the path that does not draw, for exactly this reason. **The settle order is
what covers that gap**, and it was written down nowhere: taking the deepest
first means that by the time a reader draws, what it reads has drawn and
propagated.

The trace that shows it, on seed 2 of `tests/generated.rs`: with the deepest
first and with an arbitrary pick, the two are identical event for event until a
`Span` reports its size -- 147 px one way and 317 px the other, from the same
child sizes. It had reused a subtree exactly, and inside that subtree sat a
`SetSize` whose declared width had changed and which had not been drawn yet.
24 widgets end up wrong.

So the order is not about cost, and this is the thing to fix before changing
it. Adding the missing check to `try_reuse` does make any order correct -- all
six generated cases and the hundred-seed sweep pass with the dirty set taken in
whatever order it yields. But `dirty_size_under` walks the size-dependency
subtree on every reuse that hands a size back, and that costs about what the
sort saved: at 130 of 260 widgets dirty, 7.83M instructions per frame against
8.23M; at 32 dirty, 3.32M against 2.84M, so *worse* where the dirty set is
small. Two other shapes measured and rejected on the way: putting the check at
the top of `try_reuse` rather than on the two paths that return a size (7% dearer
again), and phrasing it as `size_is_invalid`, which also lets a resize mark
through and takes the resize phase from 7M to 106M instructions per frame.

**What would actually pay is a maintained count** rather than a walk: each
widget holding how many dirty widgets sit under it through size-dependency
edges, incremented up the reader chain when a mark is added and decremented
when one is consumed. That is O(depth) at a mark and O(1) at the check, where
today it is O(dirty x depth) per widget settled. It wants agreeing first: every
place that inserts into or removes from `needs_redraw` has to pair with it, and
a count that is too low is a stale size rather than a slow frame.

`3f7cd82` takes most of it without touching the order: a widget's depth is
known where it is drawn -- its parent's plus one -- so `Painter` carries it and
`ActiveData` keeps it, and the choice reads a field instead of walking an
ancestry. Being reused counts as being visited, so the two reuse paths keep it
current; only a subtree nothing looked at can hold an old one, and nothing
under an unvisited subtree is being ordered. `depth` asserts the kept value
against the ancestry in debug builds, and the hundred-seed sweep passes with
those assertions on, reshuffles included -- those being what moves a widget to
another parent. Same load at 130 of 260 dirty: 8.16M instructions per frame to
7.14M, median 0.813 ms to 0.639, and the choosing from 25.8% of the frame to
4.7%.

What is left of it is iterating the dirty set, which a `HashSet` walks by
capacity rather than by length. Ordering it -- a `BTreeSet` keyed by the kept
depth, or a bucket per depth -- would take that too, but `needs_redraw` lives
on `Widgets` and is inserted from places with no view of the tree, so either
means a second structure inside the render state kept in step with it. For 4.7%
that is not obviously worth the coupling.

**Going order-free costs more than the order now does.** Measured 2026-09-14
with the depth carried, instructions per frame, on the same three loads:

| | 32 dirty | 130 dirty | resize |
| --- | --- | --- | --- |
| ordered, no check (what is in) | 2.80M | 7.14M | 6.70M |
| ordered, with the check | 2.83M | 7.18M | 6.72M |
| arbitrary, with the check | 3.12M | 7.38M | 6.28M |

So the check is nearly free where the order already makes it redundant, and
dropping the order costs 11% on a small dirty set. **Keeping the answer does
not help: it never hits.** The dirty set moves about once per check -- every
widget drawn takes its own mark out -- so an answer worked out against one
state of it is asked for against another, and a generation stamp invalidates
everything each time. Only a count maintained up the reader chain as marks come
and go would answer in O(1) without an invalidation to lose, and that is the
version still unbuilt.

One thing not understood: the arbitrary-order rows above are consistently ~6%
cheaper with the kept answer in than without it, though the kept answer never
hits and only two lines differ between the builds. Nothing here rests on that
difference, but it means those two rows are worth re-measuring before anyone
builds on them.

**A frame that dirties many widgets at once was not being checked, and it is
where the settle order shows.** `77bb75e` adds two generated cases -- every
declared size changing together, and a spread of widgets marked for redraw
together, where nothing changes and so no box may move -- and `IRIS_PHASE=many`
to the rig, with `IRIS_DIRTY` widgets marked per frame. At 130 of 260 widgets,
**choosing which dirty widget to settle next is 24.5% of the frame**: the dirty
set is scanned once per widget settled, and a `HashSet` is walked by capacity
rather than by length. Memoizing the depth walk within one scan does not pay
(it trades parent lookups for memo lookups, 4% more instructions); the fix is
to stop rescanning, which changes the order widgets settle in and is hers to
agree first.

**What a frame is made of now**, `perf record` on the `many` phase at 130 of
260 dirty, which is the heaviest thing the rig has: `Layers::write` 16%,
parley's `break_all_lines` 15%, `Painter::glyphs` 14%, `InstanceList::push` 7%,
`LayerDraws::free` 6%, `draw_inner` 5.5%, choosing the next dirty widget 4.7%.
Four fifths of it is primitive bookkeeping, and none of that has had a pass.
That is where to look next rather than at the settle loop.

The remaining costs after this are:

- **Writing a glyph instance per glyph per draw.** With shaping and placement
  kept, `Painter::glyphs`, `Layers::write` and `InstanceList::push` are most of
  what is left. A cold frame writes 48,050 instances for a tree that
  holds 10,872. What reaches the GPU is 10,872, since a layer uploads whole and
  `set_instance` cancels a dirty mark when the bytes are unchanged -- so this is
  CPU cost only, but a one-leaf repaint still uploads 5,863 instances where the
  pre-#16 code uploaded 106.
- **A container measures a child by drawing it in a box it will not keep.**
  The remaining-region trial removes many mismatches, but an unknown child's
  measured length can still make its final box differ, and nested containers
  compound those redraws. The general form of the shaping store is a widget
  answering from a box it has already drawn in, which -- unlike a size answer --
  is realized by definition; the cost is retaining more than one drawing's
  primitives, which is why text, where only the shaped layout has to be kept,
  is where it is worth doing first.

## The random trees

`iris::random` grows a seeded tree -- spans in every direction holding two to
four children, stacks, scrolls on either axis, alignment on either axis,
padding with each of its four sides its own number,
rects with varying opacity, text both wrapping and overflowing, a declared size
over half of it, stopping at a depth. `examples/random.rs` draws one
(`IRIS_SEED`, `IRIS_DEPTH`). `tests/generated.rs` grows each seed twice -- once
and then changed, once with the change built in -- and compares every widget's
box across eight scenarios: a size change, a resize, both, and five ways of
changing what a span holds. `a_long_run_of_seeds_agrees` is the ignored sweep,
100 seeds across all eight, 800 comparisons, about four minutes.

The property is that laying a tree out again lands where growing it cold does,
which is the same thing as layout being a function of the state. It earned
itself immediately: it found the non-terminating marking, the pixel-box reuse
check and the measured-size rule above, none of which the hand-written tests
reached, and it says the result is better than what it started from -- 90 of 90
against 83 on `db1751f`.

Four things about it that are easy to get wrong:

- **Both trees must make the same widgets in the same order.** Comparison is
  index for index, so a tree that makes fewer widgets, or frees one whose id is
  then handed to the next, stops lining up at the first difference and every
  comparison after it is against the wrong widget. Hence three spare leaves
  grown beside every span whether they end up in it or not, and detached
  children held until the comparison is over.
- **Attaching a spare moves it.** A widget belongs to one parent;
  `WeakWidget::upgrade` registers an add and panics with "cannot add a widget
  twice", so it is for a handle that was never added, not a second share.
- **Each shuffle asserts the tree actually changed** before comparing, or a
  case that quietly did nothing passes green.
- **A failure prints the widget's ancestry**, marking the ones that own a slot,
  because where two trees disagree is rarely where the cause is.

## Verifying a slice

```sh
cd <iris-worktree>
cargo fmt --all --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace
```

50 tests pass on #18's head. `--workspace` matters: `rig-input` is a crate of
its own.

Render checks are the last pass, not the iteration loop -- the owner asked for
that on 2026-09-14, since the layout tests cover the CPU part and the shots
cost real time:

```sh
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz --shot /tmp/out.png
./scripts/run-headless.sh tabs --replay /tmp/taps.touch --shot /tmp/out.png
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz --resize 900x1200@60Hz --shot /tmp/rs.png
```

- The reference shots are `tabs`, `view`, `minimal` and `text` at 1920x1200,
  plus `tabs` with a replay that switches to the image tab and adds two images.
  A `.touch` line is `<ms> down|move|up <x> <y>` in the output's own pixels; the
  tab strip is at y=24 and the five tabs at x = 192, 576, 960, 1344 and 1728,
  with the image tab's add button near (1836, 1116).
- **A resize is its own case**, and `--resize` is it: the output changes under
  the running app, and what it lands on must match a cold start at that size
  byte for byte. That caught both of #16's defects and nothing in `cargo test`
  can see it.
- **Run one at a time.** The rig reuses a single compositor and a single
  output, so two invocations at once resize each other's window and quietly
  screenshot the wrong thing. Two sets of shots were thrown away learning that.
- **Give a comparison worktree its own target dir.** While one was shared
  between two checkouts I got results I could not reproduce afterwards; the
  mechanism was never pinned down, so re-run any cross-checkout comparison in
  isolation before believing it.

Two drawing paths still have no shot of their own, and each needs a ui the
examples do not have, so both are throwaway examples written into the worktree
and deleted after:

1. An image alone in a layer, which is the case that failed GPU validation when
   every other test happened to have a rectangle in the same layer.
2. Six lines of 400px text, which forces the atlas to four pages and proves the
   array grew and its group was rebuilt.

`tabs` with the image replay covers rects, glyphs and images together, so that
one is an ordinary check now.

## How the work is sequenced

**Most fundamental first**, from the owner on 2026-09-13: *"please do more
fundamental changes first, such as library updates and core framework changes,
so that code only has to be written once"*, and *"should probably start adding
tests early on rather than later, so you don't have to make separate test
scripts and stuff."* So slices are ordered by how much depends on them, not by
what is nearest ready, and a slice arrives with tests rather than with a script
in `/tmp`.

**Agree a design before sending another variation of it.** The owner stopped
the fourth round of #11 with *"we should probably agree on the design here
rather than you keep submitting variations that I review"*. When a review comes
back about the shape of something rather than a defect in it, put the options
and a recommendation in front of her and implement what she picks.

**Nothing is submitted without a separate review pass** -- the installed
`pre-submit-review` skill: build clean, review the code, review the comments on
their own once the code has settled, then verify the claim by running it. The
fixes a review produces are themselves unreviewed code, so the passes repeat
until a round finds nothing. It has earned its place repeatedly: four defects
on #11 that format, clippy, tests and five headless renders had all passed, and
on #12 a regression introduced by the review's own first draft. `audit.sh` in
the skill directory prints every comment line a branch adds against a base ref;
the owner's standing complaint is verbose agent comments, and the default
verdict is delete. **Machine-specific notes do not belong in the repository** --
they live in `~/.claude/MACHINE.md` or a `this-machine-*` skill.

Other standing instructions from the owner:

- Pull Iris out even if the Rust application switchover is not accepted. No
  app, session, transcript, setup or server concepts in Iris; the dependency
  runs one way from `app/` to Iris.
- **Small, coherent PRs.** The original extraction PR was too large to review.
  A slice may be redone rather than transplanted, and need not remove every old
  feature. Non-conflicting pull requests may be open at once -- disjoint path
  sets, each branched from current `upstream/main` rather than stacked. She
  reviews small ones as they arrive and only avoids two *large* ones in flight.
- **Order by dependency, largest reach first.** On 2026-09-13: *"do the large
  reaching framework changes first so less has to be redone."*
- Do not recreate an `ai` branch in canonical Iris; the fork is the boundary.
- **Never rewrite a pushed branch.** Follow review with additive commits, and
  merge `upstream/main` in rather than rebasing when a branch falls behind.
- Respond to each review finding with a fix or a concise explanation. Do not
  add a ceremonial comment when the changed code already answers it.
- **A test has to guard something that could break again.** She deleted #14's
  test as pointless: the rename it guarded cannot regress. When a fix is
  structural, the structure is the test.
- **Say who decided a constraint.** LAYOUT.md §2's "move slots carry
  translation only" was written by an agent on 2026-09-04, was never asked for,
  and read as settled until she said *"I was not aware that an agent decided
  position slots should be translate only."* Mark an agent's own choice as one.

## What is left

**Next, and small:** `LazySpan`, the last of LAYOUT.md §2. `set_child_offset`
is no longer part of it -- a child offset is just placing the child, which
`Painter::place` now does.

Then, roughly in dependency order:

- **Built-in alignment, and probably size**, which the owner moved ahead of the
  rest on 2026-09-14. Reproduced in the harness: `.width(rel(0.5))` inside a
  `Dir::DOWN` span reports 200 of 400 and is handed the whole 400, and a `Pad`
  in between does not change that. **Do not "fix" it by reading the child's
  ortho `size_hint`** -- a `Pad` between the `SetSize` and the span has no hint
  of its own, so the declared width silently goes back to filling. It works
  only when nothing is in the way. Alignment has to belong to the widget rather
  than be discovered through whatever happens to sit on top of it.

  Two things beyond the bug argue for it. Built-in size removes `SetSize`, and
  with it a wrapper reporting one size while handing its child the whole box.
  And built-in alignment is what would let `OnResize::Translate` apply to
  centred content, which otherwise has to say `Redraw` because only its own
  draw knows where the middle was. Size is the harder half: a declared size
  beside the one `draw` returns is two sources of truth for one thing, so
  settle what each means before building it. This is independent of #18's
  retained-update fix now that `Aligned` is in its generated coverage; land
  #18 first, then design built-in alignment and size together as the next
  structural slice rather than mixing that representation change into this
  performance correction.
- **`OnResize::Translate`, which still does nothing.** The chain removed half
  its obstacle: a placed widget's slot holds the box it was offered while its
  drawing is a set of fractions of that box, so the two are no longer one
  field. What is still missing is a widget saying *where* in a bigger box its
  unchanged drawing should sit, which is the alignment work above.
- **`UiRenderState` behind `Rc<RefCell<..>>`**, queued by the owner on
  2026-09-13 as fundamental, and especially so for text.
- **`Len`, `LayoutLen` and dp.** The archive splits the type so that `rest` is
  unrepresentable where it is meaningless (a padding), and folds a density in
  at resolve time. 21 files mention `Len`, so it is wide but shallow.
- **The input restructure** -- `src/default/sense.rs` becomes `src/rsc/sense.rs`
  (308 lines to 2313), plus `core/src/event/controller.rs`, `desktop/input.rs`,
  `android/input.rs` and `sense_tests.rs`: pointer capture, drag slop and axis,
  platform cancellation, mask-aware hit testing, event timestamps. The
  archive's own `consumes` is what #12 landed, so that part transplants;
  `tests/pointer_routing.rs` is the acceptance criterion.
- Retained span, scrolling and layout placement.
- Retained paints, selection, overlays and shared UI runtime state.
- Generic desktop/Android framework hosts and reusable example/APK tooling.
- Application-owned fonts and application-named font families.
- Shared resource-handle bookkeeping and replaceable glyph-atlas buckets.
- Positioned text overflow and cluster-safe ellipsis.

Dependencies are current apart from `winit`, which stays on 0.30.12 until 0.31
leaves prerelease. `parley` 0.11.1 and `image` 0.25.10 are latest.

The archive is a reference, not a patch to apply -- it writes `Widget::draw`
against `painter.set_size`, which #16 replaced with a returned `Size`, and
lengths against `LayoutLen` and `density`, which canonical does not have.
Recreate a change on today's types, leave app-specific behaviour out, and
verify it independently.

```sh
cd /home/bob/repos/iris && git fetch upstream
git diff --stat upstream/main..origin/archive/full-extraction
```

## How the renderer works now

Current invariants, not history. Worth reading before touching `core/render`.

- **A primitive registers itself by being drawn.** The type carries its own
  WGSL, and `PrimitiveRegistry` keys ids by `TypeId`, so the kind comes from
  the type and there are no `RECT`/`GLYPH`/`TEXTURE` constants. Nothing is
  seeded, so an id depends on what a ui drew first and a ui pays only for the
  pipelines it uses.
- **Each primitive records its own draws.** `Primitive::render` makes a
  `PrimitiveRender` that states the layout its shader reads, uploads whatever
  it owns, and records its draws. `GlyphRender` owns the atlas and binds it
  once per list; `ImageRender` owns the images and binds one per instance; the
  default owns nothing and draws every instance in one call. The renderer sets
  the pipeline, the shared group, the list's data and its vertex buffer, and
  knows nothing else.
- **The shared bind group is the window, the masks and the move chain**, given
  to every draw. A mask texture would go here too. What a primitive samples is
  its own group, and a primitive that samples nothing has no such group in its
  pipeline.
- **Every binding size is stated.** A `None` minimum puts the binding on
  wgpu-core's late-sized list, which `is_ready` scans on every draw.
- **`shader/prelude.wgsl` plus one file per primitive**, because one module
  cannot declare two types at the same binding. The prelude carries only what
  every primitive uses -- window, masks, the chain walk, the vertex shader,
  `masked()` -- and its header is where binding numbers are written down.
- **A texture handle is drawn like anything else.** `Painter::primitive` takes
  `impl PrimitiveLike`: a primitive, or something that yields one and does
  whatever else drawing it needs -- a `&TextureHandle` retains its share on the
  way through, which a `Pod` primitive cannot.
- **Order within a layer means nothing**, and the widgets do not rely on it:
  `Stack` gives each child its own layer and `TextEdit` draws its view in a
  child layer above the selection rectangles.
- **Images are one texture and one bind group each**, so each drawn image is a
  draw call. The owner chose that on 2026-09-13 over packing images into arrays
  like atlas pages; a bindless `binding_array` was ruled out by Android
  support. Revisit only with her.
- **Layers are never freed** (`TODO` in `primitive/layer.rs`), so every layer a
  session creates is walked every frame thereafter. Measured at ~2ns per empty
  layer per frame, which is why it is the TODO's problem and not a bug of its
  own.

## Repository topology

### ai-app checkout

- `/home/bob/repos/ai-app-2`, `origin = git@git.arirex.me:iris/ai-app.git`,
  branch `rustify`.
- `iris/` is a submodule pinned at `32f6ad8`, the complete extracted snapshot,
  and `.gitmodules` points at the **bot fork**, not canonical Iris.
- Do not change either casually: ai-app needs the complete snapshot while
  canonical Iris is only partly caught up. Reconcile when canonical contains
  what ai-app needs, or when the owner accepts a temporarily non-building pin.

### standalone Iris checkout

- `/home/bob/repos/iris`, `origin` = fork, `upstream` = canonical.
- Fork `main` and `origin/archive/full-extraction` both name `32f6ad8`, the
  target snapshot. `history/full` names the source-history result `a615bcd`.
- **Do not reset, overwrite or force-push fork `main`**: it is both the target
  reference and the commit ai-app pins.
- Start each new branch from current `upstream/main` in its own worktree:

```sh
cd /home/bob/repos/iris && git fetch upstream
git worktree add -b split/19-name /home/bob/repos/iris-pr19 upstream/main
```

`/home/bob/repos/iris-pr18` is the live one. Every other `iris-pr*` worktree
holds a merged branch; they are readable references, not places to build.
`/home/bob/repos/iris-perf2` is detached at `b1b3eca`, built, and is the
"before" side of any measurement of the last two commits; `iris-size-old` is
`43ce8c7` with the old patched counters, and is how the pre-#16 column above
was measured. Copy `tests/revision_cost.rs` into either to compare.

## Cautions

- Read `/home/bob/repos/ai-app-2/AGENTS.md` and the machine-wide rules first.
  Anything about this machine -- the GPU that comes and goes, measuring a small
  performance difference, the emulator -- is in `~/.claude/MACHINE.md` and the
  `this-machine-*` skills, and belongs there rather than here.
- Keep Iris generic: session drivers, transcripts, setup and server concepts,
  app icons and product fonts stay in ai-app. Android and desktop code is Iris
  work only when it is a generic host or platform integration.
- Preserve the dirty-worktree rule. All worktrees were clean at handoff;
  anything found later may be the owner's or another agent's.
- Do not delete the archived snapshot or the fork `main` ai-app pins.
- A complete target branch is not permission to recreate the giant PR.
- Another agent was freeing disk on this VM and removed `target/` from the
  `iris-pr*` worktrees once. Sources and git state were untouched. Tell peers
  before changing shared machine tooling, and expect a cold rebuild sometimes.

## Merged so far

| PR | On canonical `main` |
| --- | --- |
| #2 | Build on the current nightly (`4275314`) |
| #3 | Request a frame after resize (`936fbdd`) |
| #4 | Decouple `iris-core` from winit (`465e430`) |
| #5 | Use vsync by default (`ec2b5d4`) |
| #6 | Notify winit before presenting (`db9b0f2`) |
| #7 | Keep unsafe reference helpers internal (`0191f20`) |
| #8 | Initialize the window uniform from the surface (`6e271e8`) |
| #9 | Preserve primitive-count recursion (`b90c855`) |
| #10 | Text layout and rendering on Parley (`0f6a28b`) |
| #11 | Atlas as an array texture, and the primitive rendering overhaul (`b234497`) |
| #13 | Build on wgpu 30 (`00d2230`) |
| #14 | Rename the `Sized` widget to `SetSize` (`32b1038`) |
| #15 | Run a ui without a window, and test one (`c8ac669`) |
| #12 | Route pointer input per kind (`43ce8c7`) |
| #16 | Size a widget while drawing it, not in a pass of its own (`f942385`) |
| #17 | Bring the headless rig into the repository (`ca2b4b2`) |

URLs are `https://git.arirex.me/iris/iris/pulls/{number}`.