WIP: Pad outside, Inset inside, and a report read raw

`Pad` moves its child's box in rather than shrinking it, `Inset` is the old
behaviour under a new name, and `in_parent_frame`'s composition and the
`reports_of` argument are gone -- a report comes up raw and the parent says
what it is a fraction of, which `Inset` does for itself.

Not landed. Everything passes except the new `Inset` test: a child declaring
`rel(0.5)` under an inset comes out 47.5 px wide of the 190 inside rather
than 95, and I have not accounted for where the second halving is. The
`Pad` half is green on its own -- the three tests that changed to `.inset()`
were using padding as scaffolding -- but landing it without a working
`Inset` would break every `.pad()` that meant inset.

core/src/fixed.rs

@@ -109,6 +109,18 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         })
     }
 
+    /// The first step at or above `v`, where [`Self::from_f32`] takes the
+    /// nearest one and is below it half the time. For a bound that has to
+    /// admit the value it came from: a measurement rounded down is a bound
+    /// that leaves out the thing it was measured from.
+    pub const fn ceil_from_f32(v: f32) -> Self {
+        let nearest = Self::from_f32(v);
+        match nearest.to_f32() < v {
+            true => nearest.next_up(),
+            false => nearest,
+        }
+    }
+
     /// From a number as it is written in source -- `16`, `1.5` -- which is
     /// the other place a value enters the grid.
     pub fn from_num(v: impl UiNum) -> Self {
@@ -372,6 +384,12 @@ impl<const SHIFT: u32> FixedVec2<SHIFT> {
         Self::new(Fixed::from_f32(v.x), Fixed::from_f32(v.y))
     }
 
+    /// The first step at or above each part, for a measurement reported as a
+    /// box: what it occupies is not less than what was measured.
+    pub fn ceil_from_f32(v: Vec2) -> Self {
+        Self::new(Fixed::ceil_from_f32(v.x), Fixed::ceil_from_f32(v.y))
+    }
+
     pub fn to_f32(self) -> Vec2 {
         Vec2::new(self.x.to_f32(), self.y.to_f32())
     }
@@ -488,6 +506,25 @@ mod tests {
         assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
     }
 
+    /// The bound a greedy line break needs: the width it was measured at is
+    /// not on the grid, and the narrowest box the break still holds for is
+    /// the step at or above it, never the one below.
+    #[test]
+    fn a_ceiling_never_lands_below_the_number_it_came_from() {
+        let step = 1.0 / (1 << PX_SHIFT) as f32;
+        for n in 0..64 {
+            let v = 189.0 + n as f32 * step / 3.0;
+            let up = Px::ceil_from_f32(v);
+            assert!(up.to_f32() >= v, "{up:?} is below {v}");
+            assert!(
+                up.to_f32() - v < step,
+                "{up:?} is more than a step above {v}"
+            );
+        }
+        // An exact step is its own ceiling.
+        assert_eq!(Px::ceil_from_f32(189.5), Px::from_f32(189.5));
+    }
+
     #[test]
     fn a_number_from_outside_is_clamped_to_the_grid() {
         assert_eq!(Px::from_f32(1e12), Px::MAX);

core/src/primitive/text.rs

@@ -107,13 +107,6 @@ impl Default for TextAttrs {
     }
 }
 
-/// How far below the longest line a width may fall and still be answered by
-/// the break in hand. A parent that offers a child the length it reported
-/// composes that length back through the box chain, so the two differ in the
-/// last bits -- and at exactly the longest line, that decides whether a line
-/// fits. Sub-pixel, so no break it admits is one a reader could see.
-const BREAK_EPSILON_PX: f32 = 0.05;
-
 /// Keeps text and its corresponding layout from getting out of sync.
 pub struct TextBuffer {
     text: String,
@@ -200,15 +193,19 @@ impl TextBuffer {
         // A greedy break at one width is the same break at every width down
         // to the longest line it produced: each line still fits, and none can
         // take a word that would not fit in the wider box. So the layout in
-        // hand already answers, and re-breaking would only be a chance to
+        // hand already answers, and re-breaking would only be work.
-        // disagree with itself -- which is what happens when a parent offers
+        //
-        // a child the length that child just reported, and the two land
+        // At the longest line exactly, with no margin below it. A narrower
-        // either side of a float.
+        // width really does break differently, so answering one from the
+        // break in hand is how a warm tree keeps lines a cold tree would
+        // never produce. The margin was here because a text reports the
+        // width it used and a parent hands that back; the report is the step
+        // at or above its longest line now, so what comes back fits.
         if let Some(key) = &self.layout_key
             && key.attrs == *attrs
             && let (Some(broke_at), Some(want)) = (key.max_width, width)
             && want <= broke_at
-            && want + BREAK_EPSILON_PX >= self.layout.width()
+            && want >= self.layout.width()
         {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::TextShapeHits);

core/src/ui/painter.rs

@@ -145,20 +145,11 @@ impl<'a> Painter<'a> {
     /// Draws a widget in a box this widget chose from the widget's own
     /// answer along the `decided` axes. On those the answer is not placed
     /// inside the box again: it already is the box, and a fraction the
-    /// widget reported of its offer, taken of this box a second time, would
+    /// widget reported, taken of this box a second time, would shrink it
-    /// shrink it twice. A container uses this where it hands back exactly
+    /// twice. A container uses this where it hands back exactly what a child
-    /// what a child asked for -- a span placing a child at the length it
+    /// asked for -- a span placing a child at the length it reported, a
-    /// reported, a scroll giving its content the content's own length.
+    /// scroll giving its content the content's own length.
-    pub fn widget_decided<'s, W: ?Sized>(
+    pub fn widget_at<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-        decided: [bool; 2],
-    ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, decided)
-    }
-
-    fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
@@ -231,10 +222,18 @@ 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()));
         }
+        // The answer as it was given. A fraction in it is a fraction of this
+        // widget's box, which is the same thing a rule beside the child
+        // means and the same thing for every box this widget hands out: a
+        // span offers each child the room left from its cursor, because a
+        // text has to wrap at the width actually there, and `rel(0.5)` is
+        // still half the span. Padding is outside what it pads for the same
+        // reason -- inset the fraction and a child's `rel` would mean the
+        // inner box while its `px` meant the outer one.
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, local, declared),
+            size,
         }
     }
 
@@ -299,7 +298,7 @@ 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, local, declared).axis(axis))
+        Some(size.axis(axis))
     }
 
     /// Whether this is the first box a child is asked about in during a draw
@@ -518,23 +517,6 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// A child's answer as lengths of the box it was asked from. A widget reports
-/// a fraction of the box it was given, and the widget that gave it wants the
-/// same length as a fraction of its own: one composition apart wherever the
-/// offer was not the whole of the parent's extent, as a span's is after a
-/// relative child. 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, local: UiRegion, declared: [Option<LayoutLen>; 2]) -> Size {
-    let mut size = size;
-    for (axis, declared) in AXES.into_iter().zip(declared) {
-        if declared.is_none() {
-            let len = local.axis(axis).len();
-            *size.axis_mut(axis) = size.axis(axis).within_len(len);
-        }
-    }
-    size
-}
-
 /// What a widget declares a length of its box to be. `leftover` is not one: a
 /// share of what is left over is only a length to the widget dividing one,
 /// so it passes up in the size instead.

core/src/ui/render_state.rs

@@ -32,7 +32,7 @@ pub(super) struct DrawInfo {
     pub offered_px: PxVec2,
     /// The axes along which the parent chose this box from the widget's own
     /// answer, so the answer is not placed inside it again. See
-    /// [`Painter::widget_decided`].
+    /// [`Painter::widget_at`].
     pub decided: [bool; 2],
 }
 
@@ -55,6 +55,9 @@ pub struct UiRenderState {
     /// Whether this frame contains a declared-length change, so any dirty
     /// dependent replaces its answer too.
     replace_answers: bool,
+    /// Widgets waiting for an ancestor to draw them, so the walk down the
+    /// depths does not pick one up again at its own depth.
+    deferred: crate::util::HashSet<WidgetId>,
     pub moves: Moves,
 }
 
@@ -68,6 +71,7 @@ impl UiRenderState {
             slots: Default::default(),
             answer_invalid: Default::default(),
             replace_answers: false,
+            deferred: Default::default(),
             moves: Default::default(),
             resized: false,
         }
@@ -192,14 +196,19 @@ impl UiRenderState {
             diag::draw_request(id, info.parent, region, info.px, info.region_node);
         }
         let align = rsc.widgets().alignment(id);
-        let replace_answer = self.answer_invalid.remove(&id)
+        // Nothing this widget has is an answer while something it measured
-            || (self.replace_answers
+        // is dirty: settling that changes what it would report, and a widget
-                && (rsc.widgets().needs_redraw.contains(&id)
+        // settled inside its parent's draw tells nobody -- the comparison
-                    || self.dirty_size_under(id, rsc.widgets())));
+        // that marks a reader is in `redraw`, which is not what asked here.
-        let retained = match replace_answer {
+        // Both retained routes are an answer, so the question is asked once
+        // rather than by each of them.
+        let stale =
+            rsc.widgets().needs_redraw.contains(&id) || self.dirty_size_under(id, rsc.widgets());
+        let replace_answer = self.answer_invalid.remove(&id) || (self.replace_answers && stale);
+        let retained = match replace_answer || stale {
             true => None,
             false => self
-                .retained_answer(id, info, rsc.widgets())
+                .retained_answer(id, info)
                 .or_else(|| self.try_reuse(id, region, info, rsc)),
         };
         let answer = retained.unwrap_or_else(|| {
@@ -489,16 +498,9 @@ impl UiRenderState {
 
     /// The answer to an ask can be retained independently of where its
     /// drawing ended up. Alignment is exactly that case: the first box is the
-    /// question and the smaller placed box holds the drawing.
+    /// question and the smaller placed box holds the drawing. Whether the
-    fn retained_answer(
+    /// answer is stale at all is its caller's question, asked once there.
-        &self,
+    fn retained_answer(&self, id: WidgetId, info: DrawInfo) -> Option<(Size, [Holds; 2])> {
-        id: WidgetId,
-        info: DrawInfo,
-        widgets: &Widgets,
-    ) -> Option<(Size, [Holds; 2])> {
-        if widgets.needs_redraw.contains(&id) || self.dirty_size_under(id, widgets) {
-            return None;
-        }
         let active = self.active.get(&id)?;
         let has_region_node = active.move_idx != active.parent_move;
         if !active.drawn
@@ -512,9 +514,11 @@ impl UiRenderState {
     }
 
     /// Whether anything whose size this widget's own size was read from is
-    /// dirty. Not needed for the answer to come right -- a changed size
+    /// dirty, which makes what it would answer not yet known. It also keeps
-    /// reaches its reader in any order -- but a reader that asks first
+    /// a reader that asks first from laying out twice, which is all it was
-    /// lays out once rather than twice.
+    /// here for while a changed size was thought to reach its reader in any
+    /// order; it does not, where the change settles inside the reader's own
+    /// draw.
     fn dirty_size_under(&self, id: WidgetId, widgets: &Widgets) -> bool {
         self.active.get(&id).is_some_and(|active| {
             active.size_deps.iter().any(|child| {
@@ -827,18 +831,33 @@ impl UiRenderState {
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
         #[cfg(feature = "layout-diagnostics")]
         let _layout = diag::timer(TimerKind::IncrementalLayout);
-        // Deepest first: a reader whose children have all settled asks each
+        // Deepest first, and strictly: a widget that cannot settle where it
-        // once, where any other order has it lay out again for whatever
+        // is defers to its parent rather than drawing the parent from
-        // settles under it afterwards. Equal-depth widgets are independent,
+        // inside itself. It marks the parent, stays marked, and waits here
-        // so their order does not matter.
+        // until the walk reaches its parent's depth.
-        while let Some(id) = {
+        //
-            let dirty = rsc.widgets().needs_redraw.iter().copied();
+        // What that buys is that nothing shallower is ever drawn while
-            dirty.max_by_key(|&id| self.depth(id))
+        // anything deeper is still dirty. A parent drawing can therefore
-        } {
+        // trust every answer it reads without descending to check whether
+        // something below is about to change it -- which is the whole class
+        // of defect where a widget settles inside its parent's draw, clears
+        // its mark there, and tells nobody its answer moved.
+        loop {
+            let next = rsc
+                .widgets()
+                .needs_redraw
+                .iter()
+                .copied()
+                .filter(|id| !self.deferred.contains(id))
+                .max_by_key(|&id| self.depth(id));
+            let Some(id) = next else { break };
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::QueuePops);
-            self.redraw(id, rsc);
+            if !self.redraw(id, rsc) {
+                self.deferred.insert(id);
+            }
         }
+        self.deferred.clear();
     }
 
     fn depth(&self, id: WidgetId) -> usize {
@@ -923,11 +942,13 @@ impl UiRenderState {
     }
 
     /// Settles a dirty widget: asks it again where its parent asked, and
-    /// tells the parent if the answer changed.
+    /// tells the parent if the answer changed. `false` where the question is
-    pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
+    /// its parent's rather than its own, which leaves it marked for the
+    /// parent to draw when the walk reaches that depth.
+    pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) -> bool {
         rsc.widgets_mut().needs_redraw.remove(&id);
         let Some(active) = self.active.get(&id) else {
-            return;
+            return true;
         };
         // Its parent resolved its declared lengths into its box and decided
         // whether to draw it at all, so a change to either is the parent's
@@ -947,14 +968,15 @@ impl UiRenderState {
                     at = self.active[&next].parent;
                 }
             }
+            // Both stay marked: the parent because it has this to draw, and
+            // this because the parent must draw it rather than keep what it
+            // has. The mark comes off in `draw_at`, where the parent draws.
             rsc.widgets_mut().needs_redraw.insert(id);
-            self.redraw(parent, rsc);
+            rsc.widgets_mut().needs_redraw.insert(parent);
-            // Whatever the parent did not draw again is nothing it holds now.
+            return false;
-            rsc.widgets_mut().needs_redraw.remove(&id);
-            return;
         }
         if !active.drawn {
-            return;
+            return true;
         }
         // Nothing above the root resolved its rules or its alignment, so its
         // box is its own to work out again against the output. Every other
@@ -969,7 +991,7 @@ impl UiRenderState {
             diag::bump(Counter::LocalRedraws);
             let old = self.remove(id, false, rsc);
             self.draw_inner(id, region, info, old, rsc);
-            return;
+            return true;
         };
         let (given_px, offered_px) = self.asked_px(id);
         // Asked again in the box its parent gave it, which is the question
@@ -979,9 +1001,8 @@ impl UiRenderState {
         // on is its lengths, so the same lengths elsewhere is one question.
         if given_px != offered_px {
             rsc.widgets_mut().needs_redraw.insert(id);
-            self.redraw(parent, rsc);
+            rsc.widgets_mut().needs_redraw.insert(parent);
-            rsc.widgets_mut().needs_redraw.remove(&id);
+            return false;
-            return;
         }
         let info = DrawInfo {
             layer: active.layer,
@@ -1014,6 +1035,7 @@ impl UiRenderState {
             }
             rsc.widgets_mut().needs_redraw.insert(parent);
         }
+        true
     }
 }
 

examples/text.rs

@@ -28,10 +28,14 @@ impl DefaultAppState for State {
             .pad(16)
             .background(panel());
 
+        // Each one takes the whole width, because `text_align` puts the
+        // glyphs somewhere in the box the text is given and a text that
+        // reports the width of its own glyphs is given exactly that.
+        let label = |text: &str, align| wtext(text).size(24).text_align(align).width(rel(1.0));
         let aligned = (
-            wtext("left").size(24).text_align(Align::LEFT),
+            label("left", Align::LEFT),
-            wtext("centred").size(24).text_align(Align::CENTER),
+            label("centred", Align::H_CENTER),
-            wtext("right").size(24).text_align(Align::RIGHT),
+            label("right", Align::RIGHT),
         )
             .span(Dir::DOWN)
             .gap(8)

src/widget/position/pad.rs

@@ -9,10 +9,10 @@ impl Widget for Pad {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         // The inner's own alignment, not the near edge. This reports the
         // inner's size plus the padding, so where the box is that answer the
-        // inset box is exactly the inner and alignment has no room to move
+        // inner is exactly what it asked for and alignment has no room to
-        // it; where the box is bigger -- a share of a row, a rule over this
+        // move it; where the box is bigger -- a share of a row, a rule over
-        // widget -- the slack is the inner's to sit in, and forcing the near
+        // this widget -- the slack is the inner's to sit in, and forcing the
-        // edge pinned it to a corner it had not asked for.
+        // near edge pinned it to a corner it had not asked for.
         let inner = painter
             .widget_within(&self.inner, self.padding.region())
             .size();
@@ -29,6 +29,45 @@ impl Widget for Pad {
     }
 }
 
+/// Room taken off the inside rather than added round the outside: the child
+/// draws in what is left once both edges are gone, and this widget is
+/// exactly as long as the box it was given.
+///
+/// So `rel(1.0)` under an [`Inset`] is the room inside it, where the same
+/// rule under a [`Pad`] is the pad's whole box and overflows it by the
+/// padding. Both are wanted; which one a layout means is which widget it
+/// reaches for.
+pub struct Inset {
+    pub padding: Padding,
+    pub inner: StrongWidget,
+}
+
+impl Widget for Inset {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let region = self.padding.inset_region();
+        let inner = painter.widget_within(&self.inner, region).size();
+        // What a fraction the child reported is a fraction of is this
+        // widget's to say, and it says the room inside: the child asked for
+        // a part of the box it drew in, and that box is shorter than this
+        // one by both edges. Then the edges go back on, so this widget is
+        // its child and the room taken off around it.
+        let (x, y) = (
+            inner.x.within_len(region.x.len()),
+            inner.y.within_len(region.y.len()),
+        );
+        Size {
+            x: LayoutLen {
+                px: x.px + self.padding.left + self.padding.right,
+                ..x
+            },
+            y: LayoutLen {
+                px: y.px + self.padding.top + self.padding.bottom,
+                ..y
+            },
+        }
+    }
+}
+
 pub struct Padding {
     pub left: Px,
     pub right: Px,
@@ -53,7 +92,24 @@ impl Padding {
             bottom: amt,
         }
     }
+    /// The box a [`Pad`] gives its child: as long as the pad's own, moved in
+    /// by the near edge. Padding is outside what it pads, so a fraction the
+    /// child asks for is a fraction of the same length whether a rule beside
+    /// it states one or it reports one, and its pixels are the same pixels.
+    /// Shrinking the box instead would make `rel` mean the inner box while
+    /// `px` meant the outer one. [`Inset`] is the widget that shrinks.
     pub fn region(&self) -> UiRegion {
+        let mut region = UiRegion::FULL;
+        region.x.start.px += self.left;
+        region.y.start.px += self.top;
+        region.x.end.px += self.left;
+        region.y.end.px += self.top;
+        region
+    }
+
+    /// The box an [`Inset`] gives its child: shorter than its own by both
+    /// edges, so what the child fills is the room left inside.
+    pub fn inset_region(&self) -> UiRegion {
         let mut region = UiRegion::FULL;
         region.x.start.px += self.left;
         region.y.start.px += self.top;

src/widget/position/scroll.rs

@@ -63,7 +63,7 @@ impl Widget for Scroll {
             region = region.offset(offset);
             region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
         }
-        painter.widget_decided(&self.inner, region, [true; 2]);
+        painter.widget_at(&self.inner, region, [true; 2]);
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it

src/widget/position/span.rs

@@ -20,9 +20,13 @@ impl Widget for Span {
                 span.flip();
             }
             let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
+            // Offered the room left from the cursor, because a text has to
+            // wrap at the width actually there, while what it reports is a
+            // fraction of the whole row: `rel(0.5)` is half the span
+            // whatever else is in it and wherever this child sits.
             let len = match painter.known_len(child, axis, region) {
                 Some(len) => len,
-                None => painter.widget_within(child, region).len(axis),
+                None => painter.widget_at(child, region, [false; 2]).len(axis),
             };
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
@@ -122,7 +126,7 @@ impl Widget for Span {
             // Along the row this box is the child's own answer, so the answer
             // is not placed in it again; across it the child sits where its
             // alignment says.
-            let placed = painter.widget_decided(child, region, [axis == Axis::X, axis == Axis::Y]);
+            let placed = painter.widget_at(child, region, [axis == Axis::X, axis == Axis::Y]);
             if shrinks {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -35,7 +35,7 @@ impl Widget for Stack {
             // child is handed a box that owes nothing to its own answer, and
             // where it sits in one bigger than itself is its own business.
             match sizing == Some(i) {
-                true => painter.widget_decided(child, region, [true; 2]),
+                true => painter.widget_at(child, region, [true; 2]),
                 false => painter.widget_within(child, region),
             };
         }

src/widget/text/mod.rs

@@ -55,8 +55,13 @@ impl TextView {
         // line up to the one it was made at: each line still fits, and none
         // could take a word that did not fit in the wider box. A line too
         // long to fit at all says nothing about narrower boxes.
+        //
+        // The step at or above that longest line rather than the nearest
+        // one, since the shaper measures in floats: the nearest step is
+        // under the line half the time, and a range starting there admits a
+        // box the line does not fit in, where the break is not this one.
         if let Some(width) = width {
-            painter.holds(Axis::X, Px::from_f32(text.size.x).min(width)..=width);
+            painter.holds(Axis::X, Px::ceil_from_f32(text.size.x).min(width)..=width);
         }
         text
     }
@@ -78,7 +83,12 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::px(tex.size);
+        // The step at or above what the shaper measured, so a parent that
+        // hands back the length this reports hands back a box the longest
+        // line fits in. Rounded to the nearest step it is half the time a
+        // hair under that line, and the break made in it is not the break a
+        // cold layout makes there.
+        let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
         let within = region.within(&painter.region());
         painter.glyphs(tex, within);
         (region, size)

src/widget/trait_fns.rs

@@ -12,6 +12,16 @@ widget_trait! {
         }
     }
 
+    fn inset(self, padding: impl Into<Padding>) -> impl WidgetFn<Rsc, Inset> {
+        // Room taken off the inside, where `pad` adds it round the outside:
+        // this is as long as the box it is given and the child fills what is
+        // left of it.
+        |state| Inset {
+            padding: padding.into(),
+            inner: self.add_strong(state),
+        }
+    }
+
     fn align(self, align: impl Into<Align>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         // An axis left out keeps whatever it had, which is centered unless
         // something else set it.

tests/cases/layout.rs

@@ -20,13 +20,13 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
-/// A drawn child reports a fraction of the box it was given, and a span
+/// A span offers each child the room left after the one before, because a
-/// offers each child what is left after the one before. So a nested span
+/// text has to wrap at the width actually there, but reads what the child
-/// that takes half of the half it was offered has taken a quarter of the row,
+/// reports as a fraction of the whole row. So two children asking for half
-/// and what follows starts three quarters along -- not at the end, which is
+/// each take the whole row between them, however much of it was left when
-/// where adding its report straight into the cursor put it.
+/// each was asked, and a third overflows.
 #[test]
-fn a_span_reads_a_child_report_as_a_fraction_of_what_it_offered() {
+fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
     let mut h = Harness::new((400, 100));
     let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
     let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
@@ -35,18 +35,60 @@ fn a_span_reads_a_child_report_as_a_fraction_of_what_it_offered() {
     h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
 
     // The nested span is placed at the length it reported and drawn there
-    // once more; half of that final box is what its child takes, packed at
+    // once more; half of that final box is what its own child takes.
-    // the nested span's own start.
+    assert_corners!(h, nested, (200, 0), (400, 100));
-    assert_corners!(h, nested, (200, 0), (300, 100));
+    assert_corners!(h, inner, (200, 0), (300, 100));
-    assert_corners!(h, inner, (200, 0), (250, 100));
+    assert_corners!(h, tail, (400, 0), (500, 100));
-    assert_corners!(h, tail, (300, 0), (400, 100));
 }
 
-/// The same reading through a pad: its inset is the whole box less the
+/// The same fraction either way round: after a 100 px child in a 400 px row,
-/// padding, so half of the inset plus the padding is half the box plus one
+/// `rel(0.5)` is 100 to 300 whether the child's own rule says so or the child
-/// padding, not two.
+/// drew half of what it was offered and reported that. Half the row, not half
+/// of the 300 px left of it.
 #[test]
-fn a_pad_reports_a_fraction_of_its_inset_as_a_fraction_of_its_box() {
+fn a_reported_fraction_is_of_the_row_like_a_declared_one() {
+    let mut declaring = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut declaring.rsc);
+    let declared = rect(Color::GREEN).width(rel(0.5)).add(&mut declaring.rsc);
+    declaring.set_root((head, declared).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(declaring, declared, (100, 0), (300, 100));
+
+    let mut reporting = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut reporting.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut reporting.rsc);
+    let reported = (inner,).span(Dir::RIGHT).add(&mut reporting.rsc);
+    reporting.set_root((head, reported).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(reporting, reported, (100, 0), (300, 100));
+}
+
+/// What the fraction a child reports is of and what box it is offered are
+/// two different lengths, and only the first is the whole row: a text still
+/// wraps at the room actually left after its neighbour, so the same
+/// paragraph is taller where less of the row is left for it.
+#[test]
+fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
+    let paragraph = "Wrapping shapes one source into as many lines as the box \
+                     leaves room for, so a paragraph's height is an answer.";
+    let height_after = |head_width: i32| {
+        let mut h = Harness::new((400, 400));
+        let head = rect(Color::RED).width(head_width).add(&mut h.rsc);
+        let text = wtext(paragraph).size(16).wrap(true).add(&mut h.rsc);
+        h.set_root((head, text).span(Dir::RIGHT).width(rel(1.0)));
+        let region = h.region(&text).unwrap();
+        (region.bot_right.y - region.top_left.y).to_f32()
+    };
+
+    let (crowded, whole_row) = (height_after(300), height_after(0));
+    assert!(crowded > whole_row, "{crowded} against {whole_row}");
+}
+
+/// Padding is outside what it pads, so a fraction under one is a fraction of
+/// the box the padding is measured from: half of a 400 px row is 200, and
+/// the pad is that plus both edges. Inset it instead and `rel` would mean the
+/// inner box while `px` meant the outer one, which is the one thing a length
+/// may not do.
+#[test]
+fn a_pad_is_outside_the_fraction_its_child_asked_for() {
     let mut h = Harness::new((400, 100));
     let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
     let padded = (inner,).span(Dir::RIGHT).pad(10).add(&mut h.rsc);
@@ -55,8 +97,23 @@ fn a_pad_reports_a_fraction_of_its_inset_as_a_fraction_of_its_box() {
     // placed inside it by its own alignment, which is not what is under test.
     h.set_root((padded, tail).span(Dir::RIGHT).width(rel(1.0)));
 
-    assert_corners!(h, padded, (0, 0), (210, 100));
+    assert_corners!(h, padded, (0, 0), (220, 100));
-    assert_corners!(h, tail, (210, 0), (310, 100));
+    assert_corners!(h, tail, (220, 0), (320, 100));
+}
+
+/// The other half of the pair: an inset takes its room off the inside, so it
+/// is exactly as long as the box it was given and the fraction its child
+/// asked for is a fraction of what is left inside. Half of the 380 left in a
+/// 400 px row is 190, and the inset is the whole 400.
+#[test]
+fn an_inset_is_inside_the_fraction_its_child_asked_for() {
+    let mut h = Harness::new((400, 100));
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let inset = (inner,).span(Dir::RIGHT).inset(10).add(&mut h.rsc);
+    h.set_root((inset,).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, inset, (0, 0), (200, 100));
+    assert_corners!(h, inner, (10, 0), (200, 100));
 }
 
 #[test]
@@ -197,7 +254,7 @@ fn a_moved_subtree_takes_its_children_with_it() {
     let mut h = Harness::new((400, 400));
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
-    let row = inner.pad(10).height(40).region_node().add(&mut h.rsc);
+    let row = inner.inset(10).height(40).region_node().add(&mut h.rsc);
     // 80 of fixed rows in a 400 window, so the span takes 80 and sits in the
     // middle of what it was given.
     h.set_root((first, row).span(Dir::DOWN));
@@ -240,7 +297,7 @@ fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
     // impossible to take out of: recovering a fraction of a box needs a
     // relative extent, and it has none on that axis.
     let inner = rect(Color::BLUE).add(&mut h.rsc);
-    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let row = inner.inset(10).height(40).add(&mut h.rsc);
     let filler = rect(Color::GREEN).add(&mut h.rsc);
     // This column is an item in a row, so it takes the width left for it
     // rather than asking for a full row-width in addition to the bar.

tests/cases/retained.rs

@@ -462,7 +462,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
-    h.set_root((padded, below).span(Dir::DOWN).pad(12));
+    h.set_root((padded, below).span(Dir::DOWN).inset(12));
     assert_corners!(h, below, (12, 132), (388, 388));
 
     h.rsc[leaf].size = Size::px((100, 200).into());

tests/cases/unsettled.rs

@@ -7,7 +7,8 @@
 //! neither: one box length, composed two ways, landing either side of the
 //! boundary that decided whether a child was drawn at all, and two boxes
 //! reached through a region node's own entry rather than through the offer
-//! that node was given.
+//! that node was given. The last is a wrapping text handed back the width
+//! it measured, rounded to a step below the line it measured there.
 
 use iris::harness::Harness;
 use iris::prelude::*;
@@ -554,3 +555,62 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
     }
     assert!(wrong.is_empty(), "{}", wrong.join("\n"));
 }
+
+const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \
+    box leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// Eight widgets, shrunk from a 118-widget tree (seed 1121, depth 4,
+/// `shuffle-swap-for-three`). The stack takes its size from the span above,
+/// the span takes its width from the longest line of the texts in it, and
+/// the text below the span is then wrapped at that width -- so a width the
+/// shaper measured comes back to it as the box to break in.
+fn plant_a_measured_width(h: &mut Harness, swapped: bool) -> (WeakWidget<Span>, WidgetId) {
+    let first: StrongWidget = rect(Color::YELLOW).add_strong(&mut h.rsc);
+    let mut inner = Span::empty(Dir::UP);
+    inner.children = match swapped {
+        true => swapped_in(h),
+        false => vec![first],
+    };
+    let inner = inner.height(142).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![inner.add_strong(&mut h.rsc), text.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::DOWN).width(195));
+    (inner, text.id())
+}
+
+/// What the span holds once its children have been swapped, which is what
+/// the warm tree is changed to and what the cold one is grown with.
+fn swapped_in(h: &mut Harness) -> Vec<StrongWidget> {
+    let paragraph = |h: &mut Harness| -> StrongWidget {
+        wtext(PARAGRAPH).size(16).wrap(true).add_strong(&mut h.rsc)
+    };
+    vec![
+        paragraph(h),
+        rect(Color::YELLOW).add_strong(&mut h.rsc),
+        paragraph(h),
+    ]
+}
+
+/// A text handed back the width it measured breaks there the way it broke
+/// when it measured it. The width the shaper answers is not on the grid, and
+/// a report rounded to the nearest step is under the longest line half the
+/// time: a warm tree then keeps a break made in a wider box while a cold one
+/// makes a narrower break in the same box, and the paragraph gains a line.
+#[test]
+fn a_text_is_given_back_a_box_the_line_it_measured_fits_in() {
+    let mut warm = Harness::new((900, 1200));
+    let (inner, text) = plant_a_measured_width(&mut warm, false);
+    warm.frame();
+    warm.rsc[inner].children = swapped_in(&mut warm);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (_, cold_text) = plant_a_measured_width(&mut cold, true);
+    cold.frame();
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+}