ai-app/docs/LAYOUT_LOG.md

Layout findings log

What the sessions reviewing Iris's retained layout found, kept so that nothing here is rediscovered. Each entry says who found it and when. Delete this file when #19 lands and its fixes are in; what must outlive it (settled design, the measurement method) belongs in docs/LAYOUT.md, and the current plan is in docs/HANDOFF.md.

Naming and logic sweep (2026-09-19)

Settled with Bryan across one session, on the branch past 58ce74d. Nothing here changed what layout computes: the cold dump is byte-identical to 58ce74d at every commit.

How a description is said. A PlaceDescAxis is built by chaining off the value that says it -- UiSpan::within_desc/shifted_desc, Len::as_desc -- never by a constructor naming the type, because a constructor sends the reader back to the start of the line. The _desc suffix is what says which type comes out. PlaceDescAxis::on_axis(axis) lifts one axis into a pair with the whole box across it; on alone was rejected as contentless and reserved for events. from_axes is the constructor taking a function, beside the from_axis taking one axis and two values.

Arithmetic that needed a comment became a name. UiSpan::place was the aligned-placement rule written out three times; LayoutLen::without_leftover was the sibling apply_leftover never had, at six sites; is_px, is_only_leftover and declared name field comparisons the surrounding comments had to translate; Holds::covers was interval containment by hand. Seven module-level functions became methods on the value each took first.

Every pair is a struct of two per-axis values, read with [axis]. LayoutHolds was four two-element arrays, so none of its own operations could be written once; it is AxisHolds on x and y, and and, covers and contains lost their loops. impl_axis_index! gives every pair Index<Axis>/IndexMut<Axis>, replacing eighteen axis/axis_mut methods -- const_index keeps them usable in const context. The bare [Option<LayoutLen>; 2] became Declared of Option<Len>, which makes "a share is never a declaration" structural rather than two filters and a comment.

Two findings in the logic, both one mistake. A value computed from other state was being stored as if it were state, and in both cases the visible symptom was something that looked like an off-by-one:

• A Span carried start as a third accumulator beside fixed and taken, assigned at three points, when every assignment was reached(fixed, taken). Both ends of a slot are now read where they are used; the variable and two of the three calls per child go, and the gap added after the last child derives nothing rather than needing to be subtracted.
• The measuring loop's cursor added px and rel by hand where the placing loop below said fixed += len.without_leftover() -- the same sum, one of them named.

One property that held but nothing guarded. A Scroll's draw writes amt and snap_end, so a second draw at another viewport reads what the first wrote. Warm matches cold only because re-clamping is idempotent and monotone. The seed scans build Scrolls and never scroll one, so this was untested; a_scrolled_view_resized_lands_where_a_cold_layout_puts_it scrolls four distances, one past the end, then widens. It passes.

Follow-up implementation review (2026-09-19)

The ask/place split, window-unit frames, exact validity preimages, and bottom-up dirty settling implement the settled design. Keep this approach. It does not guarantee one body call per widget: an unhinted descendant that reports leftover weight still needs a room ask and a slot ask. The explicit measurement redesign remains deferred until an app screen justifies it.

The fixes below are on layout/one-ask in /home/bob/repos/iris:

• A collapsed share advances both span cursors. The regression covers one and two collapsed children in all four directions.
• A masking widget owns a mask reference and reclaims its existing slot on redraw. Primitives retain their own references. Removing the mask, undrawing its owner, freeing the widget, and replacing the root release ownership; a changed inherited mask rejects drawing reuse. Tests check actual primitive mask indices, movement with and without a region node, child draw counts, clip removal/addition, and empty-mask slot reuse.
• A further handover defect: draw_inner saved the old parent only after a redraw replaced ActiveData. The old parent therefore kept the child in its list and could undraw the subtree after its new parent drew it. Capture the old parent before replacing the record. A branch-switch regression reproduces disappearing content when its new parent owns a region node, and also checks the ordinary reuse path.

The mask and handover tests fail on the reviewed code and pass with the fixes. The mask fix preserves child reuse rather than redrawing descendants on every mask repaint. No naming sweep or rounding-policy change is included.

Validation: workspace tests with and without diagnostics, the release fast oracle, and all three prescribed seed scans pass. Comparing 34,488 cold boxes against cadfba0 finds 650 changes; withholding just the collapsed-slot fix reproduces the baseline exactly. This is an expected geometry correction, not a cost-only change whose dump should remain identical. The tabs example and an exact 400 px collapsed-share fixture were rendered and inspected.

Two test-harness savings leave the random stream and coverage unchanged: generated constructs one plan per seed for its sixteen scenarios, and the warm/cold comparison constructs its diagnostic ancestry lookup only after finding a mismatch. No overall speedup is claimed; no deep profile was run.

Integration is larger than an API rename. The app's pinned 32f6ad8 has 45 commits not reachable from this review branch. In particular, the app's nested/shape masks and shared Ui ownership are absent here: #19's mask is still a single rectangle and set_mask rejects nested masks. Keep the app pin until those existing capabilities have been integrated. The "Masks" and "UI ownership" sections of LAYOUT.md describe the app-side implementation, not everything already present on the upstream review branch.

Original review of #19 at cadfba0 (2026-09-19)

The original review read cadfba0, then the tip of layout/one-ask. cargo fmt --all --check, clippy -D warnings and the 123-test suite were clean there. These are the original failures, fixed by the follow-up above.

A span misplaces the slot after a collapsed leftover child

src/widget/position/span.rs:101 keeps two cursors while it places: fixed, everything taken so far, and start, where the next slot begins. The branch that drops a share child with no room to divide advances fixed by the gap and not start:

if len.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares {
    painter.undraw(child);
    fixed.px += self.gap;
    continue;              // `start` still excludes this gap
}
let from = start;

In a 400 px row with gap(10) over children of 200 px, leftover(1) and 180 px -- exactly full, so nothing is left over and the share collapses -- the tail is placed at (215, 0)..(395, 100). Its slot was 210..400, a gap too long and a gap too early, and the declared 180 was then centred in it. The row reports a total that ends at 400. A child drawn with rel lengths is stretched into the extra gap instead of being centred in it, because the slot fills.

Recomputing the cursor in that branch fixes it, and the tail lands at (220, 0)..(400, 100):

start = shared(fixed, taken, total.leftover, room);

The 123-test suite passes with the line in. Nothing in it or in the generated oracle catches the defect: warm and cold layouts are wrong identically, so an oracle comparing the two cannot see it. This is the sharp form of the handoff's older "a vanished child leaves a double gap" item, which described the accounting and not the misplacement.

A reused child keeps the mask its parent replaced

ActiveData::parent_mask is recorded and documented as the inherited mask "the one a redraw of it must not be handed back", but try_reuse (core/src/ui/render_state.rs:565) never compares it with info.mask: it gates on dirtiness, layer, move parent and region-node status only. Painter::set_mask pushes a fresh MaskIdx on every draw, so a masking widget that redraws while its child is reused leaves that child's primitives naming a mask nobody updates again:

frame 1: masked mask=Id(0)  inner mask=Id(0)
frame 2 (the masked widget alone marked): masked mask=Id(1)  inner mask=Id(0)
frame 3 (the subtree moves up to y=10):
   mask 0: y starts at 50    <- what the inner's primitives are clipped by
   mask 1: y starts at 10    <- the live one, clipping nothing

The child's drawing is then clipped 40 px too high and its top is cut off. main guarded this with active.mask == mask in its reuse gate and re-marked the owners of a rebuilt mask in remask_shape_users; the rewrite dropped both.

Returning None from try_reuse where active.parent_mask != info.mask fixes the repro and keeps the suite green, but it redraws the whole subtree whenever a masking ancestor redraws. The better fix is to give set_mask a per-widget mask slot kept across redraws, the way UiRenderState::move_slot already keeps a move entry, so the index is stable and reposition goes on updating the one the descendants name.

Regression coverage

Both paths now have regressions in tests/cases/layout.rs and tests/cases/retained.rs; the follow-up above records the additional cases. The descriptions above preserve the original failure at cadfba0.

Clarity, in the order worth doing

Everything about naming is done: the unswept extent, the two same-typed boxes on ActiveData and Painter's four holds accumulators in 5642f20, Part::All in aeb60e5, and Place/Part themselves in 58ce74d. The settled vocabulary and the ask API are in docs/LAYOUT.md.

The clarity sweep of 3da1c71 and 7e2b4cd closed the first three items that stood here. Both are cold-dump identical to 6c84b6f, so none of it moved a box:

• Answer {size, holds} and Drawn {answer, drawing_holds} replace (Size, LayoutHolds) and the three-tuple with two LayoutHolds in it. try_reuse answers bool rather than Option<()>.
• Span::along is Span::slot; far is row, shares is has_room beside a named any_leftover, and reached guards on the weight it divides by rather than on the numerator.
• DrawInfo::px was the rel base in pixels, and all three readers printed it as the box the widget drew in. Removed; each reads region.to_px(window). Placing::window existed only to feed it.
• diag::outside holds the 23 counter lines that were inside try_reuse.
• ActiveData::is_region_node replaces four copies of move_idx != parent_move; Axis::BOTH replaces AXES in three modules; Len::rel_min, rel_max and the unused select_len are gone.

What is left, none of it urgent:

• widget_at does three linear scans per child (children.contains, under.iter_mut().find, depend_on), so a span of n children is O(n^2) per draw. Not a problem at today's sizes; it is worth knowing before a long transcript list lands on it.
• PlaceSpan is private to place.rs, so Painter::in_parent asks five yes/no questions (stated_rel_base, is_sized, within_span, narrows_rel_base, does_fill) about a three-case enum instead of matching it. Two of those questions are redundant with one another -- Sized always states a rel base -- but only because nothing constructs a Sized without one, which nothing states. pub(crate) on the enum would let in_parent read as its three cases.
• DrawInfo and ActiveData both carry placed and asked, two PlaceDesc fields distinguished only by position in every literal. They are genuinely different and documented, but the names are past participles with no operand; a rename is Bryan's vocabulary call.