Rename rest to leftover

The length kind that asks for a part of what is left once the fixed
lengths are taken is called leftover: Len::leftover(2), Len::LEFTOVER,
Size::LEFTOVER, Len::leftover the field, and apply_leftover. It says
what it is where "rest" reads as "the remainder of the list" as often as
"the remaining space", and every agent who has touched this has reached
for a third word for it.

Locals called rest that meant a region or a widget are renamed with it,
since the word now names something else.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/retained.rs

@@ -43,11 +43,11 @@ fn counted(h: &mut Harness, size: Size, dependence: OnResize) -> (WeakWidget<Cou
     (id, Counts(draws))
 }
 
-/// A fixed-width leaf beside one that takes the rest, so changing the first
-/// hands the second a different box without the output changing.
-fn pair(h: &mut Harness, rest: OnResize) -> (WeakWidget<Counted>, Counts, WidgetId) {
+/// A fixed-width leaf beside one that takes what is left over, so changing
+/// the first hands the second a different box without the output changing.
+fn pair(h: &mut Harness, leftover: OnResize) -> (WeakWidget<Counted>, Counts, WidgetId) {
     let (first, _) = counted(h, Size::from((100, 200)), OnResize::Translate);
-    let (second, draws) = counted(h, Size::REST, rest);
+    let (second, draws) = counted(h, Size::LEFTOVER, leftover);
     h.set_root((first, second).span(Dir::RIGHT));
     (first, draws, second.id())
 }
@@ -119,7 +119,7 @@ fn a_span_relays_out_when_a_child_it_measured_changes() {
 fn a_repaint_that_keeps_its_size_does_not_relay_out() {
     let mut h = Harness::new((400, 200));
     let (first, draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
-    let (second, _) = counted(&mut h, Size::REST, OnResize::Translate);
+    let (second, _) = counted(&mut h, Size::LEFTOVER, OnResize::Translate);
     h.set_root((first, second).span(Dir::RIGHT));
     let settled = draws.get();
 
@@ -158,7 +158,7 @@ impl Widget for FromHint {
         let mut region = UiRegion::FULL;
         region.y.end = region.y.start.offset(len.px);
         painter.widget_within(&self.inner, region);
-        Size::REST
+        Size::LEFTOVER
     }
 }
 
@@ -214,7 +214,7 @@ impl Widget for ReadsWidth {
 #[test]
 fn a_resize_does_not_redraw_what_the_shader_can_move() {
     let mut h = Harness::new((400, 200));
-    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Scale);
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, OnResize::Scale);
     h.set_root(leaf);
     let settled = draws.get();
 
@@ -236,7 +236,7 @@ fn a_resize_does_not_redraw_what_the_shader_can_move() {
 #[test]
 fn a_resize_redraws_what_does_not_scale() {
     let mut h = Harness::new((400, 200));
-    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, OnResize::Redraw);
     h.set_root(leaf);
     let settled = draws.get();
 
@@ -430,8 +430,8 @@ fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
     let mut h = Harness::new((400, 200));
     // What a transcript row is: something whose shaping depends on the width
     // it is given, beside something that only has to be the right shape.
-    let (wraps, wrap_draws) = counted(&mut h, Size::REST, OnResize::Redraw);
-    let (backing, back_draws) = counted(&mut h, Size::REST, OnResize::Scale);
+    let (wraps, wrap_draws) = counted(&mut h, Size::LEFTOVER, OnResize::Redraw);
+    let (backing, back_draws) = counted(&mut h, Size::LEFTOVER, OnResize::Scale);
     let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
     let bar = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((bar, row).span(Dir::RIGHT));
@@ -457,8 +457,8 @@ fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
     // drawing the child in its own box is only an answer for that box.
     let (counter, draws) = counted(&mut h, Size::from((80, 200)), OnResize::Redraw);
     let fixed = counter.width(80).add(&mut h.rsc);
-    let (rest, _) = counted(&mut h, Size::REST, OnResize::Scale);
-    let row = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    let (leftover, _) = counted(&mut h, Size::LEFTOVER, OnResize::Scale);
+    let row = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
     let bar = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((bar, row).span(Dir::RIGHT));
     let settled = draws.get();