# 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 `77bb75e`, twenty-two commits, workspace tests passing, fmt and clippy clean. The last four are the text shaping work, three retained-layout details that claimed something they did not do, and the many-widgets-dirty cases. 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: ```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//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. 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 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 ` down|move|up ` 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>`**, 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}`.