Record the resize change, and why the position chain moved up

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -9,14 +9,15 @@ Canonical `main` is **`43ce8c7`** (#12, pointer routing). Fourteen slices are
 in.
 
 - **#16** `split/16-draw-size`, worktree `/home/bob/repos/iris-pr16`, head
-  `9520996`. A widget sizes itself while drawing; `SizeCtx`, the `desired_*`
+  `984f482`. A widget sizes itself while drawing; `SizeCtx`, the `desired_*`
   methods and the size cache are gone, and `on_resize(axis)` drives the
-  retained path. One review round answered: `Widget::draw` returns the `Size`
+  retained path. Two review rounds answered: `Widget::draw` returns the `Size`
   (no `Painter::set_size`), both `place` methods are folded back into
   `widget_within`/`draw_inner`, `Painter::size_hint` records the size
-  dependency `place` used to, and `()` sizes itself rest so it is a gap.
-  `tabs` (all five tabs, and a replay that adds two images), `view` and
-  `minimal` render byte-identical to `upstream/main`.
+  dependency `place` used to, `()` sizes itself rest so it is a gap, and a
+  resize now moves the drawing instead of wiping the tree. `tabs` (all five
+  tabs, and a replay that adds two images), `view` and `minimal` render
+  byte-identical to `upstream/main`.
 - **#17** `split/17-headless-rig`, worktree `/home/bob/repos/iris-pr17`, head
   `deb9c1b`. The compositor script, `rig-input`'s `replay-touch` and the
   `.touch` parser, so a rendering claim no longer has to be checked from
@@ -25,11 +26,21 @@ in.
   disjoint from #16.
 
 **Waiting on the owner:** whether size dependencies should be recorded per
-axis. They could be, but `highest_reader`'s only trigger is `needs_redraw`,
-which carries no axis, so the precision would be unusable until the trigger
-changes: redraw a child first, compare its new size to the old, and propagate
-only to readers of the axes that moved. Offered on #16 as its own slice, ahead
-of the position chain.
+axis. They could be, but `mark_readers`' only trigger is `needs_redraw`, which
+carries no axis, so the precision would be unusable until the trigger changes:
+redraw a child first, compare its new size to the old, and propagate only to
+readers of the axes that moved. Offered on #16 as its own slice, ahead of the
+position chain.
+
+**What the resize change taught, and why the chain moved up.** `mov` cannot
+stretch a drawing out of a box with no relative extent: `UiScalar::within` puts
+a part into such a box as a plain offset from its start, so the part's own
+fraction is gone, and `outside` asks `lerp_inv` to invert a zero-length range --
+where `div_or`'s fallback returns a rel of 0 rather than saying it cannot. The
+remap then silently leaves the drawing its old size. #16 makes `OnResize::Scale`
+refuse that case, which costs a redraw of any fixed-size subtree whose box later
+changes length. Separating the drawn box from the offered one is what removes
+the cost, so the position chain now sits directly after built-in alignment.
 
 Check for a review before starting anything, and read the newest
 `submitted_at` rather than the first result: