Retain request dependencies only when discovery supplies the answer

tests/revision_cost.rs

@@ -1,19 +1,22 @@
-//! What a resize frame costs and what it holds, on a tree the revision before
-//! #16 also builds.
-//!
-//! Deliberately written in the API subset `43ce8c7` and this branch share, so
-//! the same source can be dropped into an old worktree and measured there:
-//! that is the only like-for-like comparison with the code the retained
-//! layout replaced. The random tree cannot carry one, because the generator
-//! itself changed with the work.
+//! Text-layout workloads with stable paragraphs for comparisons across revisions.
+//! PR #19's base uses the older spelling of the fixed 40-pixel width and has
+//! no diagnostics. The random generator changed with layout, so it cannot
+//! provide the same workload across the full PR.
 //!
 //!     ROWS=40 FRAMES=500 cargo test --release --test revision_cost \
 //!       -- --ignored --nocapture resize_cost
+//!     PHASE=edit ROWS=40 FRAMES=2000 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture text_updates_cost
 //!     ROWS=2000 cargo test --release --test revision_cost \
 //!       -- --ignored --nocapture text_memory
 //!
-//! Wall time on this machine varies with CPU frequency; take the number from
-//! `perf stat -e instructions:u` on the test binary directly.
+//! `text_updates_cost` selects idle, repaint, edit, or scroll with `PHASE`.
+//! It alternates a short suffix for edits so later frames do not get a longer
+//! paragraph than earlier ones. These are CPU fixtures, with no GPU submission.
+//!
+//! Use repeated `perf stat -e instructions:u` runs on the executable directly;
+//! process totals include font loading and the cold frame, so compare identical
+//! row and frame counts. Wall time on this machine is not a stable comparison.
 
 use iris::harness::Harness;
 use iris::prelude::*;
@@ -137,9 +140,10 @@ fn resize_cost() {
         println!("paragraph {at}: {:?}", h.region(id));
     }
 
-    // Two widths in turn is the friendly case for anything that remembers an
-    // answer, so `SWEEP=1` never repeats one -- a drag rather than a toggle.
+    // The sweep cycles 256 widths, avoiding the two-width cache-friendly case.
     let sweep = env("SWEEP", 0_usize) != 0;
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
     let mut elapsed = Vec::with_capacity(frames);
     for frame in 0..frames {
         let narrower = match sweep {
@@ -151,6 +155,11 @@ fn resize_cost() {
         h.frame();
         elapsed.push(start.elapsed().as_secs_f64() * 1000.0);
     }
+    #[cfg(feature = "layout-diagnostics")]
+    print!(
+        "{}",
+        iris::core::layout_diagnostics::take().per_frame(frames)
+    );
     elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
     println!(
         "resize: {frames} frames, min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
@@ -203,3 +212,64 @@ fn text_memory() {
     }
     report("after settling");
 }
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn text_updates_cost() {
+    let rows = env("ROWS", 40_usize);
+    let frames = env("FRAMES", 1000_usize);
+    let phase = env("PHASE", String::from("edit"));
+    assert!(rows > 0 && frames > 0);
+    assert!(["idle", "repaint", "edit", "scroll"].contains(&phase.as_str()));
+    let mut h = Harness::new(OUTPUT);
+    let mut rng = Rng(1);
+    let mut col = Span::empty(Dir::DOWN);
+    let first = wtext(words(&mut rng, 12, 52))
+        .size(16)
+        .wrap(true)
+        .add(&mut h.rsc);
+    col.push(first.add_strong(&mut h.rsc));
+    for _ in 1..rows {
+        col.push(
+            wtext(words(&mut rng, 12, 52))
+                .size(16)
+                .wrap(true)
+                .add_strong(&mut h.rsc),
+        );
+    }
+    let root = col.scrollable().add(&mut h.rsc);
+    h.set_root(root);
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
+    let original = h.rsc[first].content.to_string();
+    let alternate = format!("{original} another word");
+    let start = Instant::now();
+    for frame in 0..frames {
+        match phase.as_str() {
+            "idle" => {}
+            "repaint" => {
+                let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+            }
+            "edit" => {
+                h.rsc[first].content.clear();
+                h.rsc[first].content.push_str(if frame % 2 == 0 {
+                    &alternate
+                } else {
+                    &original
+                });
+            }
+            "scroll" => h.rsc[root].scroll(if frame % 2 == 0 { -12.0 } else { 12.0 }),
+            _ => unreachable!(),
+        }
+        h.frame();
+    }
+    println!(
+        "{phase}: {rows} rows, {frames} frames, {:.1} ms",
+        start.elapsed().as_secs_f64() * 1000.0
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    print!(
+        "{}",
+        iris::core::layout_diagnostics::take().per_frame(frames)
+    );
+}