Refine the frame-as-length step: one window unit, region nodes as translations

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

docs/HANDOFF.md

@@ -250,10 +250,20 @@ second draw back.
 
 ### 2. Make the frame a length and the box a region
 
-The frame stops being a coordinate system. Coordinates start at a region
-node (or the root) and every box under it is a region in that node's
-coordinates; a widget's frame is a *length* of the node's box, per axis, and
-is only what fractions resolve against:
+The frame stops being a coordinate system. **There is one coordinate unit,
+the window**: every box in the tree is a region in window units, a widget's
+frame is a *length* in the same units and is only what fractions resolve
+against, and a region node's move entry is a *translation* -- a region `rel
+1` long anchored at the node's box start -- rather than a box. The last
+point is forced, not chosen: a node whose unit was its frame would need its
+box expressed in frame units, which is a division of two lengths and not a
+`rel + px` length (a `.region_node()` child of a pad in a span room is the
+case). Pending Bryan's yes, 2026-09-18: it changes one cost. A node that
+moves, or scrolls, is one entry write whatever narrowed it (today only a
+narrowed node gets that; a transparent one is re-expressed), and a node
+whose box changes *length* at the same window size is re-expressed as a
+subtree (today a narrowed node is one write). The shader's `resolve_move`
+composes a `rel 1` entry as a translation already and does not change.
 
 ```rust
 pub struct Painter<'a> {
@@ -264,9 +274,12 @@ pub struct Painter<'a> {
     pub(super) frame: UiVec2,
     /// Where this widget's drawing goes, in the node's coordinates.
     pub(super) extent: UiRegion,
-    /// The node's box in pixels. The frame in pixels is `frame.to_px(node_px)`
-    /// and the box's is `extent.size().to_px(node_px)`.
-    pub(super) node_px: PxVec2,
+    /// The window in pixels: the frame in pixels is `frame.to_px(window)`
+    /// and the box's is `extent.size().to_px(window)`. Every `Holds` range
+    /// is in these pixels, so `in_parent` combines them without `through`,
+    /// and a widget's own frame reads convert once: `Holds::at(frame_px)
+    /// .through(frame)`.
+    pub(super) window: PxVec2,
     ...
 }
 
@@ -314,15 +327,15 @@ What this changes in the core, and why it is not merely more code:
 - `Painter::resolve` becomes `region.within(&self.extent)`; the second
   `within` through the frame goes.
 - `in_parent` no longer maps a child's frame holds through the frame length
-  per level: every widget under one node holds for the node's pixel box, so
-  the holds `and` directly, and only a region node maps through its box.
+  per level: every widget holds for the window's pixels, so the holds `and`
+  directly, at region nodes too.
 - `Part::of` gains one `within_len` per `From` end. `Of` and `All` are as
   they were.
 - `DrawInfo::frame`/`frame_abs`, `ActiveData::frame`/`frame_abs`, `local`
   in `Placing`, `recompose_subtree` and `asked_px` all change meaning:
-  `frame` is a `UiVec2` length, `frame_abs` goes, a node's move entry is the
-  node's frame length anchored at its box start (so `rel` inside means the
-  frame), and `asked_px` walks to the node rather than the root.
+  `frame` is a `UiVec2` length, `frame_abs` goes, a node's move entry is a
+  `rel 1` region at the node's box start, and `asked_px` becomes the window
+  size.
 - `Stack` needs a part that is "a box of this length, placed by the child's
   alignment" (`Part::Sized(Len)`, the same thing a declared length does),
   since `narrow` no longer makes the box.