iris: count text layouts, so "a delta shapes one block" is measured rather than argued

take_counters gains a fourth counter, text shapes, bumped in
Painter::render_text -- which TextView::render only reaches on a cache
miss, so it counts shapes and not requests. A draw counter cannot stand
in for it in either direction: a widget can be redrawn without
re-shaping (the layout is memoized by width) and re-shaped without any
extra draw, and re-shaping is the whole thing the per-block transcript
row exists to avoid.

With it, a_delta_into_a_long_reply_redraws_the_same_widgets_as_a_short_one
asserts the number docs/DECISIONS.md's 2026-09-06 entry actually claims:
one delta into a 100-paragraph reply shapes exactly one text layout, the
same as into a one-paragraph one. Before the split that was necessarily
O(message), since the reply was one buffer.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

iris/benches/message_list.rs

@@ -142,7 +142,7 @@ fn bench_first_frame(n: usize) {
     let start = Instant::now();
     render.update(&root, &mut rsc);
     let elapsed = start.elapsed();
-    let (draws, rewrites, moves) = render.take_counters();
+    let (draws, rewrites, moves, _shapes) = render.take_counters();
     report(
         &format!("(a) first frame, N={n}"),
         elapsed,
@@ -177,7 +177,7 @@ fn bench_scroll(n: usize, ticks: usize) {
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();
-        let (draws, rewrites, moves) = render.take_counters();
+        let (draws, rewrites, moves, _shapes) = render.take_counters();
         total_draws += draws;
         total_rewrites += rewrites;
         total_moves += moves;
@@ -245,7 +245,7 @@ fn bench_input_grows(n: usize, lines: usize) {
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();
-        let (draws, rewrites, moves) = render.take_counters();
+        let (draws, rewrites, moves, _shapes) = render.take_counters();
         total_draws += draws;
         total_rewrites += rewrites;
         total_moves += moves;
@@ -302,7 +302,7 @@ fn bench_insert_above_anchor(n: usize, inserts: usize) {
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();
-        let (draws, rewrites, moves) = render.take_counters();
+        let (draws, rewrites, moves, _shapes) = render.take_counters();
         total_draws += draws;
         total_rewrites += rewrites;
         total_moves += moves;
@@ -384,7 +384,7 @@ fn bench_expand_holds_edge(n: usize, growths: usize) {
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();
-        let (draws, rewrites, moves) = render.take_counters();
+        let (draws, rewrites, moves, _shapes) = render.take_counters();
         total_draws += draws;
         total_rewrites += rewrites;
         total_moves += moves;

iris/core/src/ui/painter.rs

@@ -191,6 +191,10 @@ impl<'a> Painter<'a> {
         width: Option<f32>,
     ) -> RenderedText {
         let density = self.state.density;
+        // Counted here rather than in `TextView::render`, which returns
+        // its memoized layout without reaching this -- so this counts
+        // shapes, not requests. `UiRenderState::take_counters`.
+        self.state.shape_count += 1;
         let ui = self.rsc.ui_mut();
         ui.text
             .render(buffer, attrs, width, &mut ui.textures, density)

iris/core/src/ui/render_state.rs

@@ -55,6 +55,9 @@ pub struct UiRenderState {
     draw_count: u64,
     region_mut_count: u64,
     mov_count: u64,
+    /// Text layouts actually computed -- bumped by `Painter::render_text`,
+    /// which `TextView::render` only reaches on a cache miss.
+    pub(super) shape_count: u64,
 }
 
 /// A move chain more than this deep would mean something else is wrong
@@ -76,17 +79,25 @@ impl UiRenderState {
             draw_count: 0,
             region_mut_count: 0,
             mov_count: 0,
+            shape_count: 0,
         }
     }
 
     /// Reads and zeroes the (draws, region_mut rewrites, move_offsets
-    /// writes) counters -- call once per frame before `update()` to
+    /// writes, text shapes) counters -- call once per frame before
-    /// measure exactly that frame, per LAYOUT.md section 8.
+    /// `update()` to measure exactly that frame, per LAYOUT.md section 8.
-    pub fn take_counters(&mut self) -> (u64, u64, u64) {
+    ///
+    /// The fourth is the one a draw count cannot stand in for: a widget
+    /// can be redrawn without re-shaping (`TextView::render` memoizes by
+    /// width) and re-shaped without any extra draw, and it is re-shaping
+    /// that the per-block transcript row exists to avoid -- see
+    /// `transcript_ui`'s `a_delta_into_a_long_reply_shapes_one_block`.
+    pub fn take_counters(&mut self) -> (u64, u64, u64, u64) {
         (
             std::mem::take(&mut self.draw_count),
             std::mem::take(&mut self.region_mut_count),
             std::mem::take(&mut self.mov_count),
+            std::mem::take(&mut self.shape_count),
         )
     }
 

iris/src/layout_tests.rs

@@ -68,7 +68,7 @@ fn an_unchanged_frame_draws_and_rewrites_nothing() {
     render.take_counters(); // discard the first, real draw
 
     render.update(&root, &mut rsc);
-    let (draws, rewrites, moves) = render.take_counters();
+    let (draws, rewrites, moves, _shapes) = render.take_counters();
     assert_eq!((draws, rewrites, moves), (0, 0, 0));
 }
 
@@ -101,7 +101,7 @@ fn scrolling_moves_in_o1_without_a_redraw() {
     // already clamped) rather than actually moving anything.
     rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-40.0);
     render.update(&root, &mut rsc);
-    let (draws, _rewrites, moves) = render.take_counters();
+    let (draws, _rewrites, moves, _shapes) = render.take_counters();
 
     // The pass condition (LAYOUT.md section 8, condition 3) is 0 draws and
     // 1 move_offsets write, independent of how many rects are in the

iris/src/widget/list.rs

@@ -1104,7 +1104,7 @@ mod tests {
                 .push_front(ListRow::new(key, w));
         }
         render.update(&root, &mut rsc);
-        let (draws, _rewrites, _moves) = render.take_counters();
+        let (draws, _rewrites, _moves, _shapes) = render.take_counters();
 
         // None of the already-visible rows (11, 12) were touched: the
         // extents for those keys are numerically unchanged, and the only
@@ -1242,7 +1242,7 @@ mod tests {
 
             rsc.ui.widgets.get_mut(&list_weak).unwrap().scroll(5.0);
             render.update(&root, &mut rsc);
-            let (draws, _rewrites, moves) = render.take_counters();
+            let (draws, _rewrites, moves, _shapes) = render.take_counters();
 
             // The visible window is a fixed ~10 rows regardless of n; an
             // O(n) regression would show up as draws/moves scaling with

iris/transcript-ui/src/lib.rs

@@ -615,7 +615,8 @@ mod apply_tests {
         screen.apply(&mut rsc, &old_items, &new_items);
         render.update(&tree, &mut rsc);
         assert_eq!(screen.take_rebuilds(), 0, "the delta path must be taken");
-        (render.take_counters().0, 0)
+        let (draws, _, _, shapes) = render.take_counters();
+        (draws, shapes)
     }
 
     /// The pass condition for docs/DECISIONS.md's per-block row: a delta
@@ -636,13 +637,25 @@ mod apply_tests {
             reply(100, "").len() > 3_000,
             "the long case must actually be a long message"
         );
-        let (short_draws, _) = cost_of_one_delta(1);
+        let (short_draws, short_shapes) = cost_of_one_delta(1);
-        let (long_draws, _) = cost_of_one_delta(100);
+        let (long_draws, long_shapes) = cost_of_one_delta(100);
         assert_eq!(
             short_draws, long_draws,
             "a delta into a 100-paragraph reply redrew {long_draws} widgets against \
              {short_draws} for a one-paragraph reply -- the earlier blocks are not being kept"
         );
+        // The half a draw counter cannot see, and the one the per-block
+        // row actually exists for: a redraw is free if the text engine
+        // hits its memo, and a re-shape is the expensive thing. One
+        // shape, whatever the message is worth -- the block the delta
+        // landed in. Before the split this was necessarily O(message),
+        // since the whole reply was one buffer.
+        assert_eq!(
+            (short_shapes, long_shapes),
+            (1, 1),
+            "a delta shaped {long_shapes} text layouts in a 100-paragraph reply and \
+             {short_shapes} in a one-paragraph one; it must be the last block and nothing else"
+        );
     }
 
     #[test]