Put place before the rel base, and say what a Place decides

An argument that is usually `None` goes last. So `widget_at` and `place_at`
take the place first, and `narrow_rel_base` after it.

`Place` and `Part` also now say which of the two boxes they decide, since
that is the question a caller has to answer to pick between them: `Part` is
the child's region, said as a part of the caller's own, and `Within`/`Fill`
is what becomes of the placement in it.

core/src/ui/painter.rs

@@ -169,30 +169,30 @@ impl<'a> Painter<'a> {
             let len = region.axis(axis).len();
             (len != Len::FULL).then(|| len.within_len(self.rel_base(axis)))
         });
-        self.widget_at(id, narrow, region_places(region))
+        self.widget_at(id, region_places(region), narrow)
     }
 
     /// Asks a child, saying what its fractions are of and where it is asked.
     ///
+    /// `place` is the child's region, per axis, said as a part of this
+    /// widget's own: see [`Place`]. The child draws once, in that region,
+    /// and its answer is placed inside it by re-expressing the drawing.
+    /// Nothing is drawn again in a box an answer chose; a container that
+    /// puts the answer somewhere else says so with [`Self::place_at`].
+    ///
     /// `narrow_rel_base` is the child's rel base, per axis, as a length of
     /// the window: a resolved share, or a box a sibling's answer decided.
     /// `None`, whole or per axis, forwards this widget's own -- which is
     /// what a container that only divides room passes, so a fraction under
     /// it means the same wherever it sits and however deeply it is nested.
     /// It only ever narrows: a length the child declares narrows it again
-    /// here whatever the caller says, and what comes of it is also the box
-    /// the child is asked in, placed in the part by the child's alignment.
-    ///
-    /// `place` is where the child is asked, per axis, as a part of this
-    /// widget's box: see [`Place`]. The child draws once, in that box, and
-    /// its answer is placed inside it by re-expressing the drawing. Nothing
-    /// is drawn again in a box an answer chose; a container that puts the
-    /// answer somewhere else says so with [`Self::place_at`].
+    /// here whatever the caller says, and what comes of it also narrows the
+    /// region, placed in the part by the child's alignment.
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
         place: [Place; 2],
+        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
     ) -> DrawResult<'s, 'a, W> {
         let narrow_rel_base = narrow_rel_base.into().unwrap_or([None; 2]);
         let region_node = self.rsc.widgets().is_region_node(id.id());
@@ -273,12 +273,12 @@ impl<'a> Painter<'a> {
     pub fn place_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
         place: [Place; 2],
+        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
     ) -> DrawResult<'s, 'a, W> {
         let narrow_rel_base = narrow_rel_base.into().unwrap_or([None; 2]);
         if narrow_rel_base.iter().any(Option::is_some) || !self.children.contains(&id.id()) {
-            return self.widget_at(id, narrow_rel_base, place);
+            return self.widget_at(id, place, narrow_rel_base);
         }
         let at = self.placing();
         self.state.place_in(id.id(), &at, place, self.rsc);

core/src/ui/place.rs

@@ -1,6 +1,7 @@
 use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
 
-/// What of a widget's own box a child is given, along one axis.
+/// A child's region along one axis, said as a part of the region the widget
+/// saying it was given.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Part {
     /// Window lengths from where the box starts, which is what a container
@@ -8,7 +9,7 @@ pub enum Part {
     /// length, so the cursor that sums those reports is one too. A moved box
     /// re-places every child by re-adding its start, exactly. A fraction
     /// here is a fraction of the window and not of the box -- the whole of a
-    /// box is `Of(UiSpan::FULL)`, not a `rel(1.0)` span.
+    /// box is [`Self::WHOLE`], not a `rel(1.0)` span.
     From(UiSpan),
     /// A part of the box in its own coordinates, which is what a container
     /// that insets one speaks: taking eleven pixels off the end needs no
@@ -42,12 +43,14 @@ impl Part {
     }
 }
 
-/// Where a child goes along one axis, as a part of this widget's box.
+/// A child's region along one axis, and what becomes of its placement in
+/// that region once it has answered.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Place {
-    /// The child's answer, aligned inside the part by the child's alignment.
+    /// The placement is the child's answer, aligned inside the region by the
+    /// child's alignment.
     Within(Part),
-    /// Exactly the part; the answer is not placed inside it again.
+    /// The region is the placement; the answer is not placed inside it again.
     Fill(Part),
 }
 
@@ -58,8 +61,8 @@ impl Place {
         }
     }
 
-    /// Whether the part is the drawing's box outright, rather than the box
-    /// the answer is placed inside.
+    /// Whether the region is the placement outright, rather than a box the
+    /// answer is placed inside.
     pub(crate) fn fills(self) -> bool {
         matches!(self, Self::Fill(_))
     }

src/random.rs

@@ -120,7 +120,7 @@ impl Widget for Branch {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, None, [Place::Within(Part::WHOLE), top])
+            .widget_at(&self.probe, [Place::Within(Part::WHOLE), top], None)
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -137,8 +137,8 @@ impl Widget for Branch {
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > threshold {
-            true => painter.widget_at(&self.wide, None, place),
-            false => painter.widget_at(&self.narrow, None, place),
+            true => painter.widget_at(&self.wide, place, None),
+            false => painter.widget_at(&self.narrow, place, None),
         };
         Size::LEFTOVER
     }

src/widget/position/scroll.rs

@@ -14,7 +14,7 @@ impl Widget for Scroll {
         let container_len = painter.px_len(self.axis);
         // Asked in the whole viewport, then put at the scrolled offset.
         let answer_len = painter
-            .widget_at(&self.inner, None, [Place::Fill(Part::WHOLE); 2])
+            .widget_at(&self.inner, [Place::Fill(Part::WHOLE); 2], None)
             .len(self.axis);
         let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
@@ -64,9 +64,9 @@ impl Widget for Scroll {
         // box, scrolled: its drawing moved there, not made again there.
         painter.place_at(
             &self.inner,
-            None,
             self.axis
                 .pair(Place::Fill(content), Place::Fill(Part::WHOLE)),
+            None,
         );
         // 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

src/widget/position/span.rs

@@ -36,7 +36,7 @@ impl Widget for Span {
                 None => {
                     let room = Place::Within(Part::From(along(cursor, far)));
                     painter
-                        .widget_at(child, None, axis.pair(room, across))
+                        .widget_at(child, axis.pair(room, across), None)
                         .len(axis)
                 }
             };
@@ -123,7 +123,7 @@ impl Widget for Span {
             let place = axis.pair(Place::Fill(Part::From(slot)), across);
             let narrow =
                 (len.leftover > Weight::ZERO && shares).then(|| axis.pair(Some(slot.len()), None));
-            let used = painter.place_at(child, narrow, place).len(!axis);
+            let used = painter.place_at(child, place, narrow).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.

src/widget/position/stack.rs

@@ -23,7 +23,7 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, None, [Place::Fill(Part::WHOLE); 2])
+                    .widget_at(child, [Place::Fill(Part::WHOLE); 2], None)
                     .size()
             }
             None => Size::LEFTOVER,
@@ -45,7 +45,7 @@ impl Widget for Stack {
                 continue;
             }
             painter.child_layer_at(i);
-            painter.widget_at(child, None, place);
+            painter.widget_at(child, place, None);
         }
         size
     }

tests/cases/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, None, [Place::Within(Part::WHOLE), top])
+            .widget_at(&self.probe, [Place::Within(Part::WHOLE), top], None)
             .len(Axis::X);
         let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, None, place),
-            false => painter.widget_at(&self.narrow, None, place),
+            true => painter.widget_at(&self.wide, place, None),
+            false => painter.widget_at(&self.narrow, place, None),
         };
         Size::LEFTOVER
     }

tests/cases/retained.rs

@@ -216,8 +216,8 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            [None; 2],
             [Place::Within(Part::WHOLE), Place::Within(Part::From(top))],
+            None,
         );
         Size::LEFTOVER
     }
@@ -877,11 +877,11 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -966,11 +966,11 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [Some(self.frame.x.len()), None],
                 [
                     Place::Fill(Part::From(self.region.x)),
                     Place::Fill(Part::From(self.region.y)),
                 ],
+                [Some(self.frame.x.len()), None],
             );
             Size::LEFTOVER
         }
@@ -1136,11 +1136,11 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Fill(Part::From(self.region.x)),
                     Place::Fill(Part::From(self.region.y)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -1224,7 +1224,6 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1232,6 +1231,7 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
                     ))),
                     Place::Fill(Part::From(UiSpan::FULL)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -1266,11 +1266,11 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    [None; 2],
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
                     ],
+                    None,
                 )
                 .size()
         }
@@ -1286,11 +1286,11 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [None; 2],
                         [
                             Place::Fill(Part::From(self.region.x)),
                             Place::Fill(Part::From(self.region.y)),
                         ],
+                        None,
                     )
                     .size(),
             );