iris: ScrollArea measures its content instead of drawing it twice

A container that probes a child's size was still doing it with a real
draw, so `Painter::measure` existed and almost nothing used it. Tracing
every draw of one streamed frame: 1,083 `Widget::draw` calls over 113
distinct widgets, the worst drawn 11 times at nesting depth 7-8, every
one of them mode `Draw` and none of them dirty. The cache was working --
each of the 11 was offered a genuinely different region, alternating
between an oversized probe box and a real one.

ScrollArea::draw was one of the two sources. It drew its whole content
at a box built from a stale hint, read the length back, corrected the
scroll position, and drew the content again where it belonged. The first
of those is now a measurement.

The other half is the measure fast path. A widget's reported size is a
function of its own state and the size it was offered, not of where it
was offered -- so an undirtied widget already drawn at a region of this
size has already answered, and `active.size` is the answer. This is the
same assumption `mov` makes one branch further down (same offered size,
therefore identical output, therefore a translation); it is only stated
as a size here rather than acted on as a move. Without it a measurement
costs a full recursive walk, which is what made the nesting compound.

A measurement now also peeks at the redraw mark instead of consuming it
-- it is not the redraw the mark asked for, and swallowing it would
leave the widget stale until something marked it again.

453 draws from 1,083, and the streamed frame is p50 1.18ms (from 1.22ms,
and 2.20ms before this run of work). The headless phone render is
byte-identical to the previous commit's on the real GPU.

What is deliberately NOT here: the same change to `Span::draw`'s phase 1,
which is the remaining 2x and which moves the layout by a few pixels.
The layout stays intact -- it is a position difference, not a broken
frame -- but which of the two is correct was not established, and the
suspicion (that phase 2 now takes `mov`, and `mov` accumulates deltas
where a redraw recomputes) points at a bug in `mov` rather than in
`Span`. Written up in docs/IRIS_TODO.md with Iris's target shape for
`Span`: no probe phase at all for `abs` children, and a `rest` child
forcing a reposition pass rather than a redraw.

core/src/ui/render_state.rs

@@ -509,14 +509,39 @@ impl UiRenderState {
         // and, with `LazySpan` setting no mask, outside the list's own bounds:
         // the doubled `Compacted:` row in docs/bench/iris-phone-v2-2026-09-06.md.
         // The same shape reaches any dirty widget an ancestor redraws first.
-        // A measurement consumes no redraw mark and takes none of the
+        // A measurement **peeks** at the mark rather than consuming it: it
-        // fast paths: it is not the redraw the mark asked for, and
+        // is not the redraw the mark asked for, and swallowing it would
-        // "already drawn at this region" would make it return without
+        // leave the widget stale until something else marked it again.
-        // reporting a size at all.
+        let dirty = if mode.measuring() {
-        let dirty = !mode.measuring() && rsc.widgets_mut().needs_redraw.remove(&id);
+            rsc.widgets().needs_redraw.contains(&id)
-        if let Some(active) = self.active.get_mut(&id)
+        } else {
+            rsc.widgets_mut().needs_redraw.remove(&id)
+        };
+
+        // What a measurement can answer without drawing at all.
+        //
+        // A widget's reported size is a function of its own state and the
+        // size it was offered -- not of where it was offered. So an
+        // undirtied widget already drawn at a region of this size has
+        // *already answered this question*, and `active.size` is that
+        // answer. This is the same assumption `mov` below already makes
+        // (same offered size, therefore identical output, therefore a
+        // translation rather than a redraw); it is only stated here as a
+        // size rather than acted on as a move.
+        //
+        // Without it a measurement costs a full recursive walk of the
+        // subtree, and since the containers that measure nest, that walk
+        // is what made one streamed frame 1,083 `Widget::draw` calls over
+        // 113 distinct widgets.
+        if mode.measuring() {
+            if let Some(active) = self.active.get(&id)
+                && !dirty
+                && active.region.size() == region.size()
+            {
+                return active.size;
+            }
+        } else if let Some(active) = self.active.get_mut(&id)
             && !dirty
-            && !mode.measuring()
         {
             // check to see if we can skip drawing first
             if active.region == region {

src/widget/position/scroll_area.rs

@@ -107,7 +107,11 @@ impl Widget for ScrollArea {
         // has been measured: a zero-length region on the first frame would
         // place the child's primitives against a box of no size.
         let hint = self.content_len.unwrap_or(container_len);
-        let used = painter.widget_within(&self.inner, self.child_region(hint));
+        // A **measurement**: this asks how long the content is, and the
+        // box it asks about is built from a hint that the answer below is
+        // about to correct. Drawing it here painted the whole content at a
+        // provisional offset and then painted it again at the real one.
+        let used = painter.measure(&self.inner, self.child_region(hint));
 
         // A child reporting `rel` means "this fraction of what I was
         // offered", and what it was offered is this scroll area -- so the

src/widget/position/span.rs

@@ -17,14 +17,17 @@ impl Widget for Span {
         let axis = self.dir.axis;
         let gap = self.gap.apply_rest(painter.density()).abs;
 
-        // Phase 1: draw each child once, at the ambient (unmodified, full)
+        // Phase 1: ask each child how long it is along the layout axis,
-        // region a size-only query used to see before this migration, to
+        // at the ambient (unmodified, full) region.
-        // learn its length along the layout axis. This paints real
+        //
-        // primitives at a provisional slot; phase 2 below places each
+        // A **measurement**, so it writes nothing: the provisional slot
-        // child for real via the normal `widget_within` dispatch, which
+        // this asks about is the whole span rather than the child's own
-        // only actually redraws it when that slot's *size* differs from
+        // share, so it is almost never where the child ends up, and
-        // this provisional one (most children: a resize, since the
+        // painting it there meant every child of every `Span` was drawn
-        // provisional slot is the whole span, not this child's share).
+        // twice -- once at a slot that was wrong by construction and once
+        // where it belongs. With containers nested that doubling
+        // compounds, which is where a streamed frame's 1,083 draws over
+        // 113 widgets came from.
         let lens: Vec<Len> = self
             .children
             .iter()
@@ -34,11 +37,13 @@ impl Widget for Span {
         let gap_total = gap * self.children.len().saturating_sub(1) as f32;
         let total = lens.iter().fold(Len::abs(gap_total), |s, &l| s + l);
 
-        // Phase 2: place each child for real, using the lengths just
+        // Phase 2: draw each child for real, using the lengths just
-        // learned -- the same arithmetic this loop always used. The cross-
+        // learned -- the same arithmetic this loop always used. This is
-        // axis length of *this* draw (used for `Span`'s own reported size
+        // the only draw a child gets; where its size is unchanged and only
-        // below) falls out of each child's real, resolved-width `used`
+        // its position moved, `draw_inner` turns it into one `move_offsets`
-        // here for free -- this is what replaces `desired_ortho`'s former
+        // write rather than a redraw. The cross-axis length of *this* draw
+        // (used for `Span`'s own reported size below) falls out of each
+        // child's real, resolved-width `used` here for free -- this is what replaces `desired_ortho`'s former
         // duplicate simulation of this same loop (see LAYOUT.md section 4).
         let mut start = UiScalar::rel_min();
         let mut ortho_len = Len::ZERO;