WIP: a widget's region stays put and its placement moves in it

The protocol split: `region` is the box a parent gives a widget -- what a
fraction it declares or reports is a fraction of, and the coordinates
every region it writes composes within -- and it is the same box on the
ask that measures and the ask that places. `placement` is what of that
region the drawing takes, chosen by the parent per axis or by the
widget's own answer and alignment.

That is what stops a fraction being resolved twice: the placing ask no
longer hands the widget its own answer as its box, so nothing under it
re-resolves against a box that came from its own report. `reports_of`
and `decided` are gone, folded into the two regions; `box_of` is gone;
`declared_box` becomes `ask_box`, which gives a rule the region's length
and takes the position from the placement.

84 of 92 suite tests pass. Five text and region-node cases still diverge
warm against cold, and three count a second widget draw where a span's
measuring ask and its placing ask give different placements.

core/src/ui/painter.rs

@@ -2,8 +2,8 @@
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
     Axis, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText, Size, StrongWidget,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight,
-    WidgetId, Widgets,
+    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2,
+    Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
         PrimitiveKind, TexturePrimitive,
@@ -17,8 +17,20 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// This widget's box, in the coordinates of `move_idx`.
+    /// The box its parent gave it, in the coordinates of `move_idx`: what a
+    /// fraction of this widget's area is a fraction of, and what every region
+    /// it writes composes within. The same box on the ask that measures and
+    /// the ask that places, which is what keeps a fraction under it from
+    /// being resolved twice.
     pub(super) region: UiRegion,
+    /// Where this widget's drawing sits inside that box, in the box's own
+    /// coordinates: `FULL` while its answer is not yet known, and the box
+    /// its answer or its parent chose once one of them has.
+    pub(super) placement: UiRegion,
+    /// Whether this draw read its placement, which makes the drawing one
+    /// that holds for that placement alone -- the way reading a length in
+    /// pixels makes it hold for that length.
+    pub(super) reads_placement: bool,
     /// That box in pixels, which its children's are a length of: threaded
     /// down from the box this widget was given rather than composed back up
     /// the chain, so every length in layout is one multiply from its
@@ -88,28 +100,37 @@ impl<'a> Painter<'a> {
         self.primitives.push(h);
     }
 
-    /// Writes a primitive to be rendered
+    /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
+        let at = self.placed();
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, at)
     }
 
+    /// Writes one somewhere in this widget's region. A widget that wants its
+    /// own box rather than the region it was given composes through
+    /// [`Self::placement`] first.
     pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
         let primitive = primitive.into_primitive(self);
         self.primitive_at(primitive, region.within(&self.region));
     }
 
+    /// `region` is in this widget's own region, as every region it writes is.
     pub fn set_mask(&mut self, region: UiRegion) {
         assert!(self.mask == MaskIdx::NONE);
         self.mask = self.rsc.ui_mut().masks.push(Mask {
-            region,
+            region: region.within(&self.region),
             move_idx: self.move_idx,
         });
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box: it gets the
+    /// same region -- the same area for its fractions to be of -- and is put
+    /// where this widget was put. What a container that is only a wrapper
+    /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_within(id, UiRegion::FULL)
+        let own = self.placement();
+        self.widget_at(id, UiRegion::FULL, [Some(own.x), Some(own.y)])
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -129,51 +150,41 @@ impl<'a> Painter<'a> {
     }
 
     /// Draws a widget somewhere within this one. `region` is in this widget's
-    /// own coordinates, and the child's declared lengths are still to be
-    /// taken from it. Where the child's drawing sits inside what it is given
-    /// is the child's alignment, applied where the child is drawn, so a
-    /// container positions a child either by handing it a box of exactly its
-    /// length or by leaving it room and letting its alignment decide.
+    /// own region, and is the child's own region: what its declared lengths
+    /// and its report are fractions of. Where its drawing sits inside that is
+    /// its own answer placed by its alignment.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, region.size(), [false; 2])
+        self.widget_at(id, region, [None; 2])
     }
 
-    /// Draws a widget in `region`, saying what the answer means.
+    /// Draws a widget in `region`, saying where in it the drawing goes.
     ///
-    /// `reports_of` is what a fraction the child reports is a fraction of, as
-    /// lengths of this widget's own box. It is the box the child was given
-    /// wherever that box is the child's whole area -- a pad's inset, a stack
-    /// child, a scroll's content -- and a span passes its own extent along
-    /// the row instead: it offers each child the room left from its cursor,
-    /// because a text has to wrap at the width actually there, while
-    /// `rel(0.5)` still means half the span wherever the child sits in it.
+    /// `region` is the child's own area: what a fraction it declares or
+    /// reports is a fraction of, and the coordinates the regions it writes
+    /// compose within. It is the same box on the ask that measures and the
+    /// ask that places, which is what stops a fraction under it being
+    /// resolved twice.
     ///
-    /// A `decided` axis is one where this box was chosen from the widget's
-    /// own answer. On those the answer is not placed inside the box again: it
-    /// already is the box, and a fraction taken of it a second time would
-    /// shrink it twice. A container uses that where it hands back exactly
-    /// what a child asked for -- a span placing a child at the length it
-    /// reported, a scroll giving its content the content's own length.
+    /// `placement` is what of that region the child's drawing takes, per
+    /// axis, wherever this widget is choosing. `None` leaves the axis to the
+    /// child's own answer and alignment, which is what
+    /// [`Self::widget_within`] passes. A span passes the whole row as the
+    /// region, so `rel(0.5)` is half the row wherever the child sits in it,
+    /// and places the child by passing the slot along its axis.
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
-        reports_of: UiVec2,
-        decided: [bool; 2],
+        placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
-        // Composing `FULL` through a box is not quite the identity in f32,
-        // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
-            false => region,
-        };
+        let (local, placement) = ask_box(region, declared, align, placement);
         let within = match local == UiRegion::FULL {
             true => self.region,
             false => local.within(&self.region),
@@ -219,7 +230,7 @@ impl<'a> Painter<'a> {
                 offer_len,
                 px,
                 offered_px,
-                decided,
+                placement,
             },
             None,
             self.rsc,
@@ -235,7 +246,7 @@ impl<'a> Painter<'a> {
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, reports_of, declared),
+            size: in_parent_frame(size, local.size(), declared),
         }
     }
 
@@ -267,20 +278,18 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the box it is about to be offered, if it can be
-    /// had without drawing it: from its hint, or from a drawing it already
-    /// has that holds for that box. `reports_of` is what a fraction in the
-    /// answer is a fraction of, as it is for [`Self::widget_at`].
+    /// A child's length in the region it is about to be offered, if it can
+    /// be had without drawing it: from its hint, or from a drawing it already
+    /// has that holds for that box.
     pub fn known_len<W: ?Sized>(
         &mut self,
         child: &StrongWidget<W>,
         axis: Axis,
         region: UiRegion,
-        reports_of: UiVec2,
     ) -> Option<LayoutLen> {
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
-        let local = declared_box(region, declared, align);
+        let (local, _) = ask_box(region, declared, align, [None; 2]);
         let first_ask = self.offer(child.id());
         if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
             active.offer_len = local.size();
@@ -302,7 +311,7 @@ impl<'a> Painter<'a> {
         for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
-        Some(in_parent_frame(size, reports_of, declared).axis(axis))
+        Some(in_parent_frame(size, local.size(), declared).axis(axis))
     }
 
     /// Whether this is the first box a child is asked about in during a draw
@@ -334,8 +343,10 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
+    /// `origin` is in this widget's own region, as every region it writes is.
     // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+        let origin = origin.within(&self.region);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
             let mut region = origin;
@@ -362,12 +373,35 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// This widget's box, in the coordinates its own primitives are written
-    /// in -- so a region composed `within` it may be drawn directly.
+    /// The box this widget's parent gave it, in the coordinates its own
+    /// primitives are written in -- so a region composed `within` it may be
+    /// drawn directly. Its own box is [`Self::placement`] of this one.
     pub fn region(&self) -> UiRegion {
         self.region
     }
 
+    /// Where this widget's drawing goes inside the box it was given, in that
+    /// box's coordinates: what its own answer took of it, or what its parent
+    /// chose for it. `FULL` on the ask that measures, since nothing has been
+    /// placed yet.
+    ///
+    /// Reading it is what says the drawing depends on it, so a widget that
+    /// positions its own content reads it and is drawn again once its box is
+    /// known, and one that fills whatever it is given never is.
+    pub fn placement(&mut self) -> UiRegion {
+        self.reads_placement = true;
+        self.placement
+    }
+
+    /// This widget's own box in the coordinates its primitives are written
+    /// in: its placement composed through the region it was given.
+    fn placed(&mut self) -> UiRegion {
+        match self.placement() == UiRegion::FULL {
+            true => self.region,
+            false => self.placement.within(&self.region),
+        }
+    }
+
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -401,20 +435,47 @@ impl<'a> Painter<'a> {
         placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
     }
 
-    /// This widget's box in pixels. Reading it makes the drawing one that
-    /// holds for this box only, until `holds` says how far it goes.
+    /// This widget's own box in pixels. Reading it makes the drawing one
+    /// that holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {
-        for (own, len) in self.own.iter_mut().zip([self.px.x, self.px.y]) {
-            if *own == Holds::ANY {
-                *own = Holds::at(len);
-            }
-        }
-        self.px
+        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// One axis of this widget's box in pixels. Prefer this to
+    /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
+        let part = self.placement().axis(axis).len();
+        let len = part.to_px(self.px.axis(axis));
+        let own = &mut self.own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len).through(part);
+        }
+        len
+    }
+
+    /// The lengths of this widget's own box on `axis` that what it is drawing
+    /// holds for -- the same primitives, in the same fractions and offsets
+    /// of the box, and the same reported size. A widget that read its length
+    /// in pixels holds for that one alone until it says otherwise.
+    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let part = self.placement().axis(axis).len();
+        let holds = holds.into();
+        debug_assert!(
+            holds.contains(part.to_px(self.px.axis(axis))),
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
+            self.label(),
+            self.id
+        );
+        // Kept as a range of the region's lengths, which is the one variable
+        // every range here is about: its own box is a part of that box, and
+        // `through` is the exact preimage of taking the part.
+        self.own[axis as usize] = holds.through(part);
+    }
+
+    /// One axis of the box this widget's parent gave it, in pixels -- what a
+    /// fraction of its area resolves against, and so what a container divides
+    /// among its children. Its own box is a part of this one.
+    pub fn region_px_len(&mut self, axis: Axis) -> Px {
         let len = self.px.axis(axis);
         let own = &mut self.own[axis as usize];
         if *own == Holds::ANY {
@@ -423,15 +484,14 @@ impl<'a> Painter<'a> {
         len
     }
 
-    /// The lengths of this widget's box on `axis` that what it is drawing
-    /// holds for -- the same primitives, in the same fractions and offsets
-    /// of the box, and the same reported size. A widget that read its
-    /// length in pixels holds for that one alone until it says otherwise.
-    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+    /// [`Self::holds`] stated about the region rather than about this
+    /// widget's own box, for a container whose drawing turns on the box it
+    /// was given rather than on the part of it it took.
+    pub fn region_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
         let holds = holds.into();
         debug_assert!(
             holds.contains(self.px.axis(axis)),
-            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its region",
             self.label(),
             self.id
         );
@@ -521,19 +581,16 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// A child's answer as lengths of the parent's own box. A widget reports a
-/// fraction, and `reports_of` is the length that fraction is of: the box the
-/// child was given wherever that is the child's whole area, and the parent's
-/// own extent wherever the box is a positional remainder, as a span's is
-/// after an earlier child. Pixels come through untouched either way, being
-/// that many pixels wherever they end up. A declared axis is already the
-/// parent's: it resolved the rule in its own box, and the rule is what the
-/// report says.
-fn in_parent_frame(size: Size, reports_of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
+/// A child's answer as lengths of the parent's own region. A widget reports
+/// a fraction of its own region, and `of` is that region as a length of this
+/// one. Pixels come through untouched, being that many pixels wherever they
+/// end up. A declared axis is already the parent's: it resolved the rule in
+/// its own region, and the rule is what the report says.
+fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
     let mut size = size;
     for (axis, declared) in AXES.into_iter().zip(declared) {
         if declared.is_none() {
-            *size.axis_mut(axis) = size.axis(axis).within_len(reports_of.axis(axis));
+            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
         }
     }
     size
@@ -562,9 +619,9 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLe
 /// Whether what a widget reported along an axis is the whole of the box it
 /// is in rather than a part to be placed inside it. A share fills, because a
 /// share is a length only to whoever divides one, and whoever did is the one
-/// that handed down this box. A declared axis does too: `declared_box`
-/// already placed it, in the parent's box, and the rule's length is what the
-/// widget reports there. And an axis the parent decided from the answer is
+/// that handed down this box. A declared axis does too: the rule already gave
+/// the region its length, and the rule's length is what the widget reports
+/// there. And an axis the parent decided from the answer is
 /// the answer already.
 pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: bool) -> bool {
     reported.leftover != Weight::ZERO || declared.is_some() || decided
@@ -612,21 +669,35 @@ pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) ->
     placed
 }
 
-/// Takes a widget's declared lengths in the box `region` is given in, since a
-/// fraction of a length means a fraction of that one, and puts what is left
-/// over on the side its alignment says. A caller that already reserved the
-/// space hands back the same length, so this is the identity for it.
-pub(crate) fn declared_box(
+/// A child's own region and where in it its drawing goes, from the box it is
+/// offered, the lengths its rules declare, and what its parent chose.
+///
+/// A rule gives the region its length outright -- that is what makes a rule
+/// win, and it is why a widget under one never learns of it -- and the region
+/// then sits where its parent placed it, or where its alignment says if its
+/// parent left the axis open. With no rule the region is the whole of what
+/// was offered, since that is the area a fraction under it is a fraction of,
+/// and what the parent chose is where in it the drawing goes. So the two
+/// coordinate spaces are the same one wherever a placement survives.
+pub(crate) fn ask_box(
     mut region: UiRegion,
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> UiRegion {
-    for (axis, len) in AXES.into_iter().zip(declared) {
-        let Some(len) = len else { continue };
+    placement: [Option<UiSpan>; 2],
+) -> (UiRegion, [Option<UiSpan>; 2]) {
+    let mut placed = [None; 2];
+    for (axis, (len, chosen)) in AXES.into_iter().zip(declared.into_iter().zip(placement)) {
+        let Some(len) = len else {
+            placed[axis as usize] = chosen;
+            continue;
+        };
         let span = region.axis_mut(axis);
         let len = Len::from_parts(len.rel, len.px);
-        span.start += (span.len() - len).scale(align.axis(axis).rel());
+        span.start = match chosen {
+            Some(chosen) => chosen.start,
+            None => span.start + (span.len() - len).scale(align.axis(axis).rel()),
+        };
         span.end = span.start + len;
     }
-    region
+    (region, placed)
 }