fix(flowchat): bound synchronous virtualizer updates

design-system/apps/design-lab/src/design-layout.css

@@ -85,6 +85,26 @@
   line-height: var(--openbitfun-type-body-md-line-height);
 }
 .lab-shell [hidden] { display: none !important; }
+/* Product-site typography belongs to the shell; previews retain their published type roles. */
+.lab-shell .lab-page.design-home,
+.lab-topbar,
+.design-home-slogan {
+  font-family: var(--_lab-font);
+}
+/* Both brand lockups share the shell's typography, with spacing and color owned locally. */
+.design-wordmark-link,
+.design-home-footer-wordmark {
+  font-size: var(--_lab-wordmark-size);
+  font-weight: var(--_lab-wordmark-weight);
+  letter-spacing: var(--_lab-wordmark-tracking);
+  line-height: var(--_lab-wordmark-leading);
+}
+/* Keep the full-bleed footer aligned with the shell column at every viewport width. */
+.design-home-footer {
+  inline-size: 100vw;
+  margin-inline: calc(50% - 50vw);
+  padding-inline: max(var(--openbitfun-space-6), calc((100vw - var(--_lab-content-width)) / 2 + var(--openbitfun-space-6)));
+}
 /* The homepage's closing band is the one block that leaves the centred column and runs edge to edge
    (see .design-home-footer). Where the platform reserves space for a scrollbar, 100vw is wider than
    what is visible, so the band is clipped instead of widening the document. clip is not a scroll
@@ -130,10 +150,6 @@
   gap: var(--openbitfun-space-3);
   color: var(--openbitfun-color-content-primary);
   text-decoration: none;
-  font-size: var(--_lab-wordmark-size);
-  font-weight: var(--_lab-wordmark-weight);
-  letter-spacing: var(--_lab-wordmark-tracking);
-  line-height: var(--_lab-wordmark-leading);
 }
 /* The app mark is silver artwork on a transparent canvas. The site flattens it to solid ink in
    its light header; the Lab does the same, and lifts it back for the dark theme. */

design-system/apps/design-lab/src/pages/OverviewPage.css

@@ -44,16 +44,6 @@
   min-width: 0;
 }
 
-/* Typography: the homepage runs the product site's own face — IBM Plex Mono, regular weight,
-   tight leading, slightly negative tracking. The topbar follows on every route, because shell
-   chrome that changed font between pages would read as a bug. Component instances inside the
-   preview frame keep the design system's published type roles, so their text stays on the sans
-   stack. Every other page still renders in the design system's own sans. */
-.lab-shell .lab-page.design-home,
-.lab-topbar {
-  font-family: var(--_lab-font);
-}
-
 /* The headline states the design stance and what it is a stance about: one sentence, no
    subtitle. Weight is the real 600 cut (see the @font-face block in design-layout.css) — the
    reference site has no 600, and a synthesised bold on a display-size mono headline looks
@@ -65,7 +55,6 @@
 .design-home-slogan {
   max-width: 60rem;
   margin: 0;
-  font-family: var(--_lab-font);
   font-size: var(--openbitfun-type-display-fluid-title-font-size);
   font-weight: var(--openbitfun-font-weight-semibold);
   line-height: var(--openbitfun-type-display-fluid-title-line-height);
@@ -320,10 +309,6 @@
    tile; if a surface-inverse token is ever added, this is the line that should switch to it. */
 .design-home-footer {
   --_lab-band-pad-block: calc(var(--openbitfun-space-12) * var(--_lab-home-air));
-  /* The band is the viewport, not the page's column, so it reproduces the column gutter in its own
-     padding: the shell's auto margin plus the page's 36px inner padding. Below the shell's width cap
-     that expression goes negative, which is what the max() floors. */
-  --_lab-column-gutter: max(var(--openbitfun-space-6), calc((100vw - var(--_lab-content-width)) / 2 + var(--openbitfun-space-6)));
   --_lab-on-dark: var(--openbitfun-color-content-on-dark);
   --_lab-on-dark-secondary: color-mix(in srgb, var(--openbitfun-color-content-on-dark) 72%, transparent);
   --_lab-on-dark-muted: color-mix(in srgb, var(--openbitfun-color-content-on-dark) 50%, transparent);
@@ -332,15 +317,11 @@
   display: flex;
   flex-direction: column;
   gap: calc(var(--openbitfun-space-12) * var(--_lab-home-air));
-  /* Full bleed: the band leaves the page's centred column and runs to both edges, so it carries no
-     radius and the page keeps no block-end padding under it. */
-  inline-size: 100vw;
-  margin-inline: calc(50% - 50vw);
+  /* Full-bleed geometry and column alignment are owned by design-layout.css. */
   /* The band opens on the page's full rhythm but closes on half of it: the small print is the last
      line on the page, so the space under it only has to hold the band's edge rather than separate
      it from anything. */
   padding-block: var(--_lab-band-pad-block) calc(var(--openbitfun-space-6) * var(--_lab-home-air));
-  padding-inline: var(--_lab-column-gutter);
   color: var(--_lab-on-dark);
   background: var(--openbitfun-color-content-on-light);
 }
@@ -372,10 +353,6 @@
   margin: 0;
   color: var(--_lab-on-dark);
   text-decoration: none;
-  font-size: var(--_lab-wordmark-size);
-  font-weight: var(--_lab-wordmark-weight);
-  letter-spacing: var(--_lab-wordmark-tracking);
-  line-height: var(--_lab-wordmark-leading);
 }
 
 .design-home-footer-wordmark-arrow {

design-system/apps/design-lab/src/pages/OverviewPage.tsx

@@ -91,10 +91,10 @@ export function OverviewPage({ onNavigate, colorScheme, contrast, density, token
   const previewSessions: readonly PreviewSessionRow[] = [
     { id: "claw", icon: "user", name: "Claw" },
     { id: "claw-session", indent: true, name: "New Claw Session" },
-    { id: "bitfun", icon: "folder", name: "BitFun" },
-    { id: "bitfun-session", indent: true, name: "New Code Session" },
-    { id: "bitfun-welcome", indent: true, name: t("design.homePreview.welcome"), selected: true },
-    { id: "bitfun-more", name: t("design.homePreview.showMore"), metadata: <NumberBadge value="+14" /> },
+    { id: "openbitfun", icon: "folder", name: "OpenBitFun" },
+    { id: "openbitfun-session", indent: true, name: "New Code Session" },
+    { id: "openbitfun-welcome", indent: true, name: t("design.homePreview.welcome"), selected: true },
+    { id: "openbitfun-more", name: t("design.homePreview.showMore"), metadata: <NumberBadge value="+14" /> },
     { id: "root", icon: "session", name: "root", metadata: <StatusPill emphasis tone="success">38.47.103.247</StatusPill> },
     { id: "root-session", indent: true, name: "New Code Session" },
     { id: "root-relay", indent: true, name: "relay-server account" },
@@ -184,7 +184,7 @@ export function OverviewPage({ onNavigate, colorScheme, contrast, density, token
                             {t("design.homePreview.contextMachine")}
                           </Button>
                           <ComposerDivider />
-                          <Button size="xs" tabIndex={-1} trailingIcon={<Icon name="chevron-down" />} variant="text">BitFun</Button>
+                          <Button size="xs" tabIndex={-1} trailingIcon={<Icon name="chevron-down" />} variant="text">OpenBitFun</Button>
                           <ComposerDivider />
                           <span className="design-home-preview-context">1.0.0-explore</span>
                           <ComposerDivider />

design-system/apps/design-lab/src/preview/MultiSurfaceMorph.css

@@ -27,7 +27,7 @@
   position: relative;
   inline-size: var(--_morph-w);
   block-size: var(--_morph-h);
-  border: var(--_lab-rule);
+  border: var(--openbitfun-border-width-default) solid var(--openbitfun-color-border-subtle);
   border-radius: var(--_morph-r);
   overflow: hidden;
   background: var(--openbitfun-color-surface-canvas);
@@ -105,7 +105,7 @@
 }
 
 .design-morph-phone-head {
-  border-block-end: var(--_lab-rule);
+  border-block-end: var(--openbitfun-border-width-default) solid var(--openbitfun-color-border-subtle);
 }
 
 .design-morph-phone-thread {
@@ -149,7 +149,7 @@
   content: "";
   position: absolute;
   inset: var(--openbitfun-space-3);
-  border: var(--_lab-rule);
+  border: var(--openbitfun-border-width-default) solid var(--openbitfun-color-border-subtle);
   border-radius: var(--openbitfun-radius-pill);
 }
 
@@ -166,7 +166,7 @@
 
 .design-morph-watch-wave > i {
   inline-size: 3px;
-  block-size: var(--_morph-wave, 40%);
+  block-size: var(--_morph-wave);
   border-radius: var(--openbitfun-radius-pill);
   background: var(--openbitfun-color-accent-default);
 }
@@ -196,7 +196,7 @@
   gap: var(--openbitfun-space-4);
   inline-size: min(calc(100% - var(--openbitfun-space-16)), 640px);
   padding: var(--openbitfun-space-3) var(--openbitfun-space-5);
-  border: var(--_lab-rule);
+  border: var(--openbitfun-border-width-default) solid var(--openbitfun-color-border-subtle);
   border-radius: var(--openbitfun-radius-2xl);
   background: var(--openbitfun-color-surface-panel);
   color: var(--openbitfun-color-content-primary);

scripts/check-core-boundaries.test.mjs

@@ -72,6 +72,31 @@ const MODULES = [
 
 const TEST_ROOT = join('C:', 'repo');
 
+test('builtin agent catalog requires the GeneralPurpose fast model default', () => {
+  const rule = requiredContentRules.find(
+    rule => rule.path === 'src/crates/execution/agent-runtime/src/agents.rs'
+      && rule.patterns.some(pattern => pattern.message.includes('GeneralPurpose')),
+  );
+  assert.ok(rule);
+  const mapping = `builtin_agent_spec(
+    "GeneralPurpose",
+    SubAgent,
+    "fast",
+    SubagentVisibilityPolicy::public(),
+  )`;
+  const catalog = `fn builtin_agent_definition_specs() {
+    ${mapping};
+    SubagentVisibilityPolicy::restricted(["Claw"]);
+  }`;
+  const accepts = text => rule.patterns.every(({ regex }) => regex.test(text));
+  assert.ok(accepts(catalog));
+  assert.ok(!accepts(catalog.replace(mapping, '')));
+  assert.ok(!accepts(catalog.replace('"fast"', '"primary"')));
+  assert.ok(!accepts(catalog.replace('SubAgent,', 'Mode,')));
+  assert.ok(!accepts(catalog.replace('"GeneralPurpose"', '"Explore"')));
+  assert.ok(!accepts(catalog.replace('SubagentVisibilityPolicy::restricted(["Claw"]);', '')));
+});
+
 test('tool-runtime search regex stays available without enabling HTML extractors', () => {
   const rule = requiredContentRules.find(
     rule => rule.path === 'src/crates/execution/tool-execution/Cargo.toml',

scripts/core-boundaries/rules/source/required-rules.mjs

@@ -7827,8 +7827,8 @@ export const requiredContentRules = [
         message: 'missing builtin agent definition catalog owner',
       },
       {
-        regex: /builtin_agent_spec\(\s*"GeneralPurpose",\s*SubAgent,\s*"primary"/,
-        message: 'missing GeneralPurpose runtime default model mapping',
+        regex: /builtin_agent_spec\(\s*"GeneralPurpose",\s*SubAgent,\s*"fast"/,
+        message: 'missing GeneralPurpose runtime fast default model mapping',
       },
       {
         regex: /SubagentVisibilityPolicy::restricted\(\["Claw"\]\)/,

src/web-ui/src/flow_chat/components/modern/FLOWCHAT_VERIFICATION.md

@@ -52,6 +52,7 @@ each missing what the other had.
 | `../../../infrastructure/diagnostics/flowChatViewportDiagnostics.test.ts` | coalescing, placement sampling, the switch |
 | `useFlowChatVirtualizer.test.ts` | the offsets-and-positions boundary |
 | `useFlowChatVirtualizer.measurement.test.tsx` | `measureRenderedItems` against a real virtualizer |
+| `useFlowChatVirtualizer.cascade.test.tsx` | bounded synchronous measurement notifications, cross-frame convergence, ordinary synchronous opening, reader takeover and StrictMode/session cleanup; real hooks with supplied geometry, not native paint evidence |
 | `useFlowChatVirtualizer.initial-window.test.tsx` | tail-first window, empty hydration, head default, one-time seed, user-scroll takeover, opening readbacks before native events, suspended viewport, measured-window reconciliation with delayed scroll/scroll-end delivery; supplied DOM geometry, not performance validation |
 | `useFlowChatVirtualizer.aim.test.tsx` | the re-aim, and giving it up on takeover |
 | `VirtualMessageList.layout.test.ts` | the item-height estimate and the spacer |

src/web-ui/src/flow_chat/components/modern/FLOWCHAT_VIRTUALIZATION.md

@@ -124,6 +124,26 @@ the saved hint, attempts, or member identities.
 
 ## Measurement compensation and cached offsets
 
+Geometry updates and React rendering have separate delivery contracts. The
+virtualizer updates its size cache, applies owner-approved compensation and
+publishes the physical offset synchronously. Its local core adapter coalesces
+render requests within a commit and permits at most 16 notification-driven
+commits before yielding to an animation frame. The frame renders current state;
+it does not retain a scroll target or replay a former owner's write. Cleanup
+cancels the frame, including session remounts and StrictMode effect replay.
+
+This bounds a converging measurement cascade as well as an oscillating one:
+many near-empty rows can require more than React's nested-update limit while
+the window replaces estimates with measurements. Do not defer only the opening
+reconciliation callback: follow layout effects and physical range clamping can
+also advance the window. Explicit `measureRenderedItems` remains synchronous
+for callers that must read geometry before the next React commit. Normal small
+cascades still settle synchronously; the budget reset alone causes no render.
+`useFlowChatVirtualizer.cascade.test.tsx` covers tiny and zero-height rows,
+convergence across frames, reader takeover and remount cleanup using real hooks
+and supplied geometry. It is not proof of the production crash's data trigger
+or native paint timing.
+
 When the viewport owner accepts a shift for a measured row wholly above the
 reader, the virtualizer publishes the actual scroll offset after updating its
 size cache, before selecting the next rendered window. This applies during