Record #16, the position chain, and why the hint shortcut is not the fix

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -6,7 +6,12 @@ Not a decisions log; delete it when the extraction is done.
 ## Where things stand
 
 Canonical `main` is **`43ce8c7`** (#12, pointer routing). Fourteen slices are
-in and nothing is open.
+in.
+
+- **#16** `split/16-draw-size`, worktree `/home/bob/repos/iris-pr16`, head
+  `f192f75`. A widget sizes itself while drawing; `SizeCtx`, the `desired_*`
+  methods and the size cache are gone, and `size_dependence(axis)` drives the
+  retained path. `tabs`, `view` and `minimal` render byte-identical to `main`.
 
 Check for a review before starting anything, and read the newest
 `submitted_at` rather than the first result:
@@ -102,6 +107,21 @@ today's `Len` and let the dp slice follow, rather than pulling both in at once.
 
 Still in the target, roughly in dependency order:
 
+- **The position chain** (LAYOUT.md §2): `UiData::move_offsets`, per-primitive
+  slot ids and a bounded chain walk in WGSL, so moving a subtree writes one
+  slot instead of every descendant's primitives. Agreed to come directly after
+  #16.
+- **Built-in alignment, and possibly size**, directly after the position
+  chain. A child of a span is handed the full extent on the ortho axis, so
+  `.width(rel(0.5))` inside a `Dir::DOWN` span changes what the child reports
+  and not the box it gets. **Do not "fix" this 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, which is the shape of bug the review pass is for.
+  Alignment has to belong to the widget rather than be discovered through
+  whatever happens to sit on top of it. Size is the harder half: a declared
+  size beside `set_size` is two sources of truth for one thing, so settle what
+  each means before building it.
 - **`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. After the