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);