Say what a leftover means where nothing divides it

Bryan's rule, generalising the max he gave for Scroll's content length on
2026-09-18: a leftover under a parent that does not divide is still a leftover
and acts as a minimum, so the length is max(box, px + rel*box) -- it fills the
rest where the fixed parts are shorter and overflows where they are longer.

Measured against a span, which implements it, and against the non-dividing path,
which drops the overflow in the one row where the fixed part is longer than the
box. Recorded with the cause and the window contract a fix needs; not fixed,
because what a length means is Bryan's to settle and nothing here mixes the two
outside the span's own test.

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

docs/HANDOFF.md

@@ -177,6 +177,12 @@ glyphs do not follow a shortened entry.
   other layout system does that; confirm it is wanted.
 - A span can overflow itself without bound, so boxes of negative length reach
   children and nothing states what a widget may assume about one.
+- A `leftover` under a parent that does not divide is a minimum size --
+  `max(box, px + rel*box)` (Bryan, 2026-09-20). A span does that; the
+  non-dividing path drops the overflow where the fixed part is longer than
+  the box. Measured in `docs/LAYOUT_LOG.md` under the ninth sweep, with the
+  window contract a fix needs. Nothing here mixes the two, so nothing is
+  wrong on screen.
 - `Fixed::div` by zero answers `MIN`/`MAX` while `ratio` answers `ZERO`; both
   are caller bugs under `debug_assert`, but the fallbacks differ.
 - `docs/LAYOUT.md` §4, §5 and the density section name `Painter::place`,

docs/LAYOUT_LOG.md

@@ -37,6 +37,45 @@ either way at `77ed7a2`. The alternative reading -- a hint narrows even under a
 share rule -- would mean changing that comment instead, and is Bryan's to
 prefer if he does.
 
+**What a leftover means where nothing divides it** (Bryan, 2026-09-20, on the
+finding above). A leftover under a parent that does not divide is still a
+leftover, acting as a minimum: where the `px` and `rel` parts come to less than
+the box it fills the rest, and where they come to more they overflow as normal.
+The length is `max(box, px + rel*box)` -- which is the same `max` he gave for
+`Scroll`'s content length on 2026-09-18, generalised to every non-dividing
+parent.
+
+Measured at `c2b8bf8`, a probe recording the box it is asked in, in a 400 px
+window, under `.wrapper()` against a one-child span:
+
+    rule                      nothing divides   a span divides
+    leftover(1)                           400              400
+    px(50) + leftover(1)                  400              400
+    px(500) + leftover(1)                 400              500
+    rel(0.5) + leftover(1)                400              400
+    px(500), no leftover                  500              500
+
+One row disagrees. A leftover whose fixed part is longer than the box loses the
+overflow where nothing divides it. The span is right and says so in place --
+"One that also asked for pixels or a fraction keeps those and overflows" -- and
+`only_a_pure_leftover_child_disappears_when_nothing_is_left` asserts it at
+100..120 of a 100 px row. The cause is that `LayoutLen::declared` refuses to
+answer for anything carrying leftover weight, so the non-dividing path never
+learns the fixed part and falls back to the offer. The same length without the
+share does overflow (drawn -50..450, its alignment centring it), so what
+swallows it is the share and not the overflow.
+
+Fixing it means answering the longer of the offer and the fixed part, which is
+not a `Len`: a maximum of two linear forms is not linear in the box, so it has
+to be resolved where the box is known and the crossover pinned as a window
+contract -- which is exactly what `Span` does with `has_room` and
+`painter.window_holds(axis, holds.through(room))`. In `widget_at` that range
+belongs to the parent, whose box decides it, so a parent of such a child would
+redraw across the crossover. **Not done: it is what a length means, and it is
+Bryan's to say whether that cost is wanted.** Nothing in the repository mixes
+`px` or `rel` with `leftover` outside the span's own test, so nothing is wrong
+on screen today.
+
 **Marking a widget for redraw had no name.** Twenty-one sites under `tests/`
 said it as `widgets_mut().get_dyn_mut(id);` with the widget thrown away: five
 with a `let _ =` in front, one with a comment explaining what the line was for