Ask each child once and place its answer by re-expression

A widget draws in the box it is asked in and its answer is placed inside
that box by re-expressing the drawing; nothing is drawn again in a box an
answer chose. The offer machinery, whose job was to tell a measuring draw
from a placing one, goes with the placing draw. A span measures each child
from its cursor and moves fixed children to their slots with place_at; a
share child is asked once more in its decided slot with its frame narrowed
to it. A stack asks non-sizing children in the box its sizing child
decided. A scroll asks its content once and moves it to the scrolled
offset. A local redraw asks the retained question again and puts the
answer back where the parent placed it.

A symbolic length a child pinned composes through Part::Of exactly where
the part is the whole box less pixels, and pins the parent's own length
otherwise; dropping it let a pad reuse a drawing across a narrowed frame
of the same pixel length (shrinker seeds 60, 248 and 384 at depth 5).

Suite 114/114 including the two decided-box pins, fast oracle 11/11,
shrinker 400 seeds at depth 5 over all fifteen cases.

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

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    TextureHandle, UiRegion, WidgetId,
+    LayerId, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -20,17 +20,22 @@ pub struct ActiveData {
     /// length. Its length is the same on every ask, which is what
     /// a local redraw relies on to ask its parent's own question again.
     pub frame: UiRegion,
-    /// What of its parent's extent the drawing was given, and what it was
-    /// given at the parent's first ask of it -- the question a cold layout
-    /// asks. A part is a length from the extent's start, so an extent that
-    /// moved re-places every child by re-adding that start.
+    /// The length its frame was narrowed to on each axis, as a length of
+    /// its parent's frame: a declared rule, or a box its parent decided for
+    /// it. `None` forwards the parent's frame whole. Kept as a length rather
+    /// than a position so that every placement puts the frame back in the
+    /// part it is given.
+    pub narrow: [Option<Len>; 2],
+    /// Where its drawing was put, as a part of its parent's box, and where
+    /// it was asked. The two differ where a container asks in one place and
+    /// places the answer in another -- a row measures from its cursor and
+    /// puts the child in its slot. A part is a length from the box's start,
+    /// so a box that moved re-places every child by re-adding that start.
     pub place: [Place; 2],
     pub offer_place: [Place; 2],
-    /// The box that ask gave it, in its frame's coordinates. Kept rather
-    /// than worked out again from where its parent's own box is now: a
-    /// parent drawn again in the box its own answer chose gives its children
-    /// boxes it never measured anything in, and the measurement this widget
-    /// answered is the one its parent's layout was built on.
+    /// The box it was asked in, in its frame's coordinates: the box its
+    /// drawing was made in and the one its contract is about. Its drawing
+    /// is placed elsewhere by re-expression, never by asking again there.
     pub offer_part: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.