Record the frame rule, the decided axes, and the root entry's removal

Iris 5b78002 on #18: a child's answer comes back in the asker's frame, the
near-edge override is per-axis decided flags, and the window is no longer
a move entry. Also records the threading plan that closes the region-node
offer defect, and why widening the grid would not.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -12,6 +12,12 @@ head is **`5f16617`**, eighty-two commits, pushed. Built-in alignment is
 complete there; see "Built-in alignment" below for the retained-layout
 details. No PR reviews were present when checked on 2026-09-15.
 
+**`5b78002` on top of it** (2026-09-16) does two things Bryan asked for after
+the frame and root questions below. A child's answer now comes back to the
+widget that asked in *that widget's* frame, and the window is no longer a
+move entry -- see "A report is a fraction of the box it was given" and the
+root item under "Fixed point". `Holds::through` is untouched by either.
+
 The current head completes LAYOUT.md §2's position chain and the requested
 `leftover` behavior. A child whose length is only `leftover` is not drawn when
 nothing is left. A child that also asks for pixels or a relative fraction keeps
@@ -551,6 +557,24 @@ across it.
   `a_length_in_pixels_is_that_many_pixels_however_it_is_nested`, over a row
   buried under three containers that are each a fraction of their parent;
   swept over 2,100 box widths and exact at every one.
+- **A report is a fraction of the box the widget was given, and comes back
+  to the asker in the asker's frame** (Bryan, 2026-09-16: `rel` has nothing
+  to do with the remainder; it is the proportion of the space the widget
+  was given). `placed_box` always read it that way. `Span` did not: it added
+  a drawn child's `rel` straight into a cursor that counts fractions of the
+  row, which is right only while the offer has the row's whole extent --
+  and a `Span` offers what is left after the cursor, so it broke once a
+  relative child preceded another. `Pad` summed padding onto a fraction of
+  its inset the same way and reported its box plus the padding for a child
+  that filled the inset. `DrawResult::size`/`len` and `known_len` now
+  compose the answer through the offer's length before handing it back
+  (`in_parent_frame`), so a container reads lengths of its own box and the
+  box `Span` hands back afterwards is the one `placed_box` chose. A
+  declared axis is excepted: the parent resolved the rule in its own box
+  and the rule is what the report says there. A hint is used as a
+  declaration and so is in the parent's frame too; `hints_agree` compares
+  it with the raw report, which holds because every hint is pixels or
+  `leftover`. A hint with a fraction in it would need one frame chosen.
 - **A length given as a share is not**, and cannot be: children asking for
   the same fraction come out one or two steps apart, 0.001 to 0.002 px. A
   position is the quantity that gets rounded, so the row fills exactly and no
@@ -641,13 +665,31 @@ box** gets its content placed twice: once by the child, once by the box around
 it. Found three times before the class was fixed rather than the instances --
 `Stack::size(Child(i))`, `Scroll`'s orthogonal axis, and `Pad`.
 
-`Painter::widget_aligned(child, region, align)` is the override: an
+`Painter::widget_decided(child, region, [bool; 2])` is the override as of
-`Option<RegionAlign>` carried in `DrawInfo` and resolved once in `draw_inner`,
+`5b78002`, and it is per axis and not an alignment: it says the parent chose
-so the root resolves like anything else. `Pad`, `Stack` and `Scroll` pass the
+this box from the child's own answer along those axes, so the answer is not
-near edge for children whose size they report. `ActiveData` keeps both the
+placed inside it again. It replaced `widget_aligned(child, region, NEAR)`,
-resolved alignment, so a local redraw asks the question its parent asked, and
+which forced the near edge on *both* axes and, being an alignment, still let
-the widget's own, which is what a change is compared against -- an override
+`placed_box` scale a fractional answer against the box -- fine while every
-means the answer is the parent's to give again.
+overriding container handed a box its child had reported in pixels, and
+wrong the moment `Span` had to use it. `Span` now decides the row axis alone
+when it places a child at the length it reported, since across the row the
+child sits where its alignment says; `Scroll` decides both, and `Stack` both
+for its sizing child. `DrawInfo::decided` and `ActiveData::decided` carry it,
+so a local redraw asks the question its parent asked; alignment is always the
+widget's own property now, and `ActiveData::align` is gone.
+
+**A fractional answer taken of the parent's second box shrinks twice.**
+Found by the frame fix's own test: a nested span reporting `rel(0.5)` was
+placed by its parent at a quarter of the row, correctly, and then
+`placed_box` took half of *that* box because the parent's second ask was not
+an override. A report of "half of what you give me" has no fixed point but
+zero, so the framework asks exactly twice -- once at the offer, once in the
+box chosen from the answer -- and the second box is final on the axes the
+parent decided. This is also why a root that reports a fraction of the
+window is placed inside it by its own alignment and then scaled to fit,
+which the two new tests in `tests/cases/layout.rs` avoid by ruling the root
+to the window; that is old behaviour and was not changed.
 
 ### Alignment as a property changed two examples, unnoticed
 
@@ -784,6 +826,16 @@ pixels and `Holds`. Decided with Bryan on 2026-09-15.
   and shrinking this allowance is the test of whether that worked. Too wide
   is the unsound side of it. **Run the generated cases in debug**, since the
   assertion is compiled out of every release run.
+- **The window is not a move entry** (`5b78002`). It was one, holding the
+  output in pixels, so composing through it zeroed every `rel` below and the
+  shader's `rel * window.dim` and the CPU's final multiply by the output
+  were both dead. Now a chain bottoms out in `MoveIdx::NONE`, the window is
+  applied where a fraction becomes pixels -- `to_px(output_size)` on the
+  CPU, the uniform in the shader -- and a resize rewrites no retained entry
+  and re-uploads nothing but the uniform. Instances were never re-uploaded
+  on a resize either way; they are `(rel, px)` in their node's frame and the
+  shader walks the chain per vertex. The cost of a resize is whatever
+  `Holds` redraws.
 - A pointer, a wheel notch, a shaped glyph advance and a window size arrive
   as floats and are put on the grid where they arrive. `Vec2` stays what the
   GPU and the platform speak; `PxVec2` is what layout decides in.
@@ -833,19 +885,42 @@ Traced, at a 900x1200 window, with `reorder` rotating both spans:
 - Marking every widget dirty converges warm onto cold, which is what says
   the retained path and not the arithmetic is at fault.
 
-**What makes the fix awkward** is coordinate frames, and it is worth knowing
+**The fix is to stop recomputing the offer at all** (agreed with Bryan,
-before starting. `ActiveData::offer` is in the parent *widget's* coordinates
+2026-09-16). `ActiveData::offer` is in the parent *widget's* coordinates and
-and `ActiveData::region` is in the parent *node's*, so "was this node placed
+`ActiveData::region` is in the parent *node's*, so "was this node placed
-where it was offered" is not a comparison either field can answer alone. Two
+where it was offered" is not a comparison either field can answer alone --
-shapes look plausible:
+and neither answer is needed. By the retained contract a drawing and its
+answer depend on the box's pixel *lengths* only; checked against every
+widget, the pixel reads are `px_len` and `px_size` and nothing reads a
+position. The offer's symbolic position was only ever an input to its pixel
+size, and that the parent already knew when it asked. So:
 
-- Compose offers through a region node by expressing the node's offer as a
+- Thread the placed box's pixel size down beside `offered_px` -- one
-  part of its placed box -- an inverse composition, with a rounding of its
+  expression per level, `local.size().to_px(parent_px)` -- and keep both on
-  own.
+  `ActiveData`. Cold layout computes them the same way, so warm and cold
-- Let `offered_region` say it cannot answer, and have `redraw` escalate to
+  agree by construction rather than by allowance.
-  the parent the way `parent_must_place` does. Correct and small, but it
+- `at_offer` becomes `px == offered_px`. Equal lengths at a different
-  costs region nodes their point for partial repaint unless the "placed
+  position no longer escalate: the widget draws at `region`, which is the
-  where offered" case is detected, which is the frame problem again.
+  `aea878d` fix without the second draw. A region node whose lengths differ
+  still draws twice, and the first box only needs the right length.
+- `redraw` reads the stored `offered_px`. `offered_region`, `px_of`,
+  `px_region`, `Moves::compose`/`size_of`, `slot_wide` and `wide.rs` go; a
+  narrow `resolve` stays for `window_region`, which is hit testing and not a
+  layout decision.
+- `remap_subtree`'s `Translate` keeps lengths, so stored sizes stay valid
+  at no cost on the hot path; `Scale` is the resize path and recomputes
+  them down the subtree from the same expression.
+- `Holds::through` becomes the exact preimage of the truncating map
+  `px + floor(rel * B)`: two integer divisions, no `ROUTES`, no `way_in`,
+  and the whole-of-a-box case is the identity. The allowance existed only
+  because the box arrived by a second route.
+
+Symbolic regions then exist for the GPU and for remaps alone, and their
+rounding is cosmetic below a step. That is the separation the fixed-point
+work kept reaching for. **Do not widen the grid instead**: every failure in
+this document is one value reached by two expressions, sitting exactly on a
+boundary because the boundary was defined by the same value coming back the
+other way, and no precision shrinks a residue that is the whole distance.
 
 ## The leftover boundary
 
@@ -1043,7 +1118,9 @@ Queued from this work, in order:
   chain in floats -- with the comment saying so, "what has to hold is that
   this agrees with itself frame to frame, not that it matches the CPU to the
   last bit" -- and then lands each edge with
-  `snap_floor(rel * dim) + snap_floor(px)`, which is a whole pixel.
+  `snap_floor(rel * dim + px)`, which is a whole pixel. (It floored the two
+  parts apart until `5b78002`, which was harmless only while the root entry
+  zeroed every `rel`.)
   **Measured: every box edge in the `tabs` render is a hard edge**, no
   partially covered column anywhere along the band, and the two `pad(10)`
   gaps are exactly ten pixels on both sides of the truncation change. So the