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 84f589e, fifteen commits, workspace tests passing, fmt and clippy clean. It is LAYOUT.md §2's position chain, generalised to boxes. /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:

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.

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 passes. The ignored 100-seed sweep passed every transition through seed 59 and now gets past the former scroll failure at seed 52; at seed 60 it reaches the already-documented iterative wrapping-text defect, differing by 0.00003 px after SwapForThree.

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. During an output resize, the resize condition stays live until these updates finish, so an Aligned ancestor chooses the new child box before a pixel-dependent descendant draws in it.
• 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.

The remaining costs are:

• 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. This is now the largest CPU layout cost.
• Redundant draws rewrite primitives. 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, so this is CPU cost only -- but a one-leaf repaint still uploads 5,863 instances where the pre-#16 code uploaded 106.

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

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:

./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.

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:

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.

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}.