fix(frontend): harden activity split responsiveness

frontend/src/index.css

@@ -220,6 +220,24 @@
     text-transform: uppercase;
     color: oklch(var(--muted-foreground));
   }
+  .thot-activity-prose,
+  .thot-activity-prose p,
+  .thot-activity-prose li,
+  .thot-activity-prose a,
+  .thot-activity-prose code,
+  .thot-activity-prose th,
+  .thot-activity-prose td {
+    overflow-wrap: anywhere;
+    word-break: break-word;
+  }
+  .thot-activity-prose table { table-layout: fixed; }
+  .thot-activity-prose pre {
+    overflow-x: hidden;
+    white-space: pre-wrap;
+    overflow-wrap: anywhere;
+    word-break: break-word;
+  }
+  .thot-activity-prose pre code { white-space: inherit; }
 }
 
 

frontend/src/shell/AppShell.session-mgmt.test.tsx

@@ -21,6 +21,23 @@ const LIST = [
 const resumeResult = (id: string, alreadyActive = false) =>
   HttpResponse.json({ id, alreadyActive });
 
+class ControlledResizeObserver {
+  static instances: ControlledResizeObserver[] = [];
+  constructor(private readonly callback: ResizeObserverCallback) {
+    ControlledResizeObserver.instances.push(this);
+  }
+  observe = vi.fn();
+  disconnect = vi.fn();
+  trigger() {
+    this.callback([], this as unknown as ResizeObserver);
+  }
+}
+
+function setContainerWidth(element: HTMLElement, width: number) {
+  Object.defineProperty(element, "clientWidth", { configurable: true, value: width });
+  act(() => ControlledResizeObserver.instances.at(-1)?.trigger());
+}
+
 function deferred() {
   let resolve!: () => void;
   const promise = new Promise<void>((onResolve) => { resolve = onResolve; });
@@ -30,8 +47,11 @@ function deferred() {
 beforeEach(() => {
   localStorage.clear();
   FakeEventSource.instances = [];
+  ControlledResizeObserver.instances = [];
   (globalThis as any).EventSource = FakeEventSource;
   (globalThis as any).PointerEvent = MouseEvent;
+  (globalThis as any).ResizeObserver = ControlledResizeObserver;
+  window.matchMedia = vi.fn().mockReturnValue({ matches: true, addEventListener: vi.fn(), removeEventListener: vi.fn() });
   useSessionStore.getState().resetSession();
   server.use(
     http.get("http://localhost:8787/sessions", () => HttpResponse.json(LIST)),
@@ -789,6 +809,7 @@ test("opens an accessible resizable activity split and persists pointer width",
   await userEvent.click(screen.getByRole("button", { name: /show model activity/i }));
 
   const shell = screen.getByTestId("app-shell");
+  setContainerWidth(shell, 1200);
   const separator = screen.getByRole("separator", { name: /resize model activity/i });
   expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("384px");
   expect(separator).toHaveAttribute("aria-orientation", "vertical");
@@ -818,6 +839,7 @@ test("resizes the activity split with keyboard and exposes responsive drawer cla
   await userEvent.click(screen.getByRole("button", { name: /show model activity/i }));
 
   const shell = screen.getByTestId("app-shell");
+  setContainerWidth(shell, 1200);
   const separator = screen.getByRole("separator", { name: /resize model activity/i });
   expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("448px");
   fireEvent.keyDown(separator, { key: "ArrowLeft" });
@@ -829,10 +851,53 @@ test("resizes the activity split with keyboard and exposes responsive drawer cla
 
   const panel = screen.getByRole("heading", { name: "Model activity" }).closest("aside");
   expect(panel).toHaveClass(
-    "fixed",
+    "static",
-    "w-[min(90vw,24rem)]",
+    "w-[var(--activity-panel-width)]",
-    "lg:static",
-    "lg:w-[var(--activity-panel-width)]",
   );
   expect(separator).toHaveClass("touch-none", "cursor-col-resize");
 });
+
+test("uses the measured app shell for the desktop activity split", async () => {
+  localStorage.setItem(ACTIVITY_PANEL_STORAGE_KEY, "576");
+  server.use(http.post("http://localhost:8787/sessions/:id/resume", () => resumeResult("s1")));
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await userEvent.click(await screen.findByRole("button", { name: /show model activity/i }));
+
+  const shell = screen.getByTestId("app-shell");
+  setContainerWidth(shell, 1200);
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("576px");
+  expect(screen.getByRole("separator", { name: /resize model activity/i })).toBeInTheDocument();
+
+  setContainerWidth(shell, 1024);
+  expect(shell.style.getPropertyValue("--activity-panel-width")).toBe("512px");
+
+  setContainerWidth(shell, 799);
+  expect(screen.queryByRole("separator", { name: /resize model activity/i })).not.toBeInTheDocument();
+  expect(screen.getByTestId("conversation-column")).toHaveClass("flex-1");
+  expect(screen.getByRole("heading", { name: "Model activity" }).closest("aside")).toHaveClass("fixed");
+});
+
+test("cancels an active resize when the measured shell becomes too narrow", async () => {
+  localStorage.setItem(ACTIVITY_PANEL_STORAGE_KEY, "576");
+  server.use(http.post("http://localhost:8787/sessions/:id/resume", () => resumeResult("s1")));
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
+  await userEvent.click(await screen.findByRole("button", { name: /show model activity/i }));
+
+  const shell = screen.getByTestId("app-shell");
+  setContainerWidth(shell, 1200);
+  const separator = screen.getByRole("separator", { name: /resize model activity/i });
+  Object.assign(separator, {
+    setPointerCapture: vi.fn(), releasePointerCapture: vi.fn(), hasPointerCapture: vi.fn(() => true),
+  });
+  fireEvent.pointerDown(separator, { pointerId: 1, clientX: 576 });
+  fireEvent.pointerMove(separator, { pointerId: 1, clientX: 400 });
+  setContainerWidth(shell, 799);
+  fireEvent.pointerUp(separator, { pointerId: 1, clientX: 300 });
+
+  expect(localStorage.getItem(ACTIVITY_PANEL_STORAGE_KEY)).toBe("576");
+  expect(shell).toHaveAttribute("data-activity-resizing", "false");
+});

frontend/src/shell/AppShell.tsx

@@ -33,7 +33,7 @@ import type { CSSProperties } from "react";
  * portal's left sidebar, and the workflow phases ride a slim strip above the chat.
  */
 export function AppShell() {
-  const { containerRef, panelWidth, resizing, separatorProps } = useActivityPanelResize();
+  const { containerRef, panelWidth, desktopSplit, resizing, separatorProps } = useActivityPanelResize();
   const activityWidthStyle = {
     "--activity-panel-width": `${panelWidth}px`,
   } as CSSProperties;
@@ -387,9 +387,9 @@ export function AppShell() {
           onResume={doResume}
         />
       )}
-      {showActivity && <ModelActivityPanel onClose={() => setShowActivity(false)} />}
+      {showActivity && <ModelActivityPanel desktopSplit={desktopSplit} onClose={() => setShowActivity(false)} />}
-      {showActivity && (
+      {showActivity && desktopSplit && (
-        <div className="relative hidden w-0 shrink-0 lg:block">
+        <div className="relative w-0 shrink-0">
           <div
             {...separatorProps}
             data-testid="activity-resize-handle"

frontend/src/shell/ModelActivityPanel.test.tsx

@@ -40,6 +40,25 @@ test("renders question, reasoning, and response in chronological order", () => {
   expect(screen.queryByText("bash")).not.toBeInTheDocument();
 });
 
+test("uses the activity-only Markdown wrapping renderer for long content", () => {
+  useSessionStore.setState({
+    activityLog: [
+      { kind: "prompt", phase: "F1", text: "https://example.test/" + "x".repeat(120) },
+      { kind: "thinking", phase: "F1", text: "Inline `" + "code".repeat(40) + "`" },
+      { kind: "assistant", phase: "F1", text: "| Cell |\n| --- |\n| " + "table".repeat(40) + " |\n\n```sql\n" + "select_".repeat(40) + "\n```" },
+    ],
+  });
+
+  render(<ModelActivityPanel onClose={vi.fn()} />);
+
+  const renderers = document.querySelectorAll(".thot-activity-prose");
+  expect(renderers).toHaveLength(3);
+  expect(renderers[0]).toHaveClass("thot-prose");
+  expect(renderers[1].querySelector("code")).not.toBeNull();
+  expect(renderers[2].querySelector("td")).not.toBeNull();
+  expect(renderers[2].querySelector("pre")).not.toBeNull();
+});
+
 test("uses an explicit default-deny activity-kind allowlist", () => {
   const predicate = (activityPanelModule as unknown as {
     isVisibleModelActivity?: (entry: ActivityEntry) => boolean;

frontend/src/shell/ModelActivityPanel.tsx

@@ -37,7 +37,7 @@ export function formatModelActivity(text: string): string {
 function MarkdownBody({ entry }: { entry: ActivityEntry }) {
   const tone = entry.kind === "thinking" ? "text-muted-foreground" : "text-foreground";
   return (
-    <div className={`thot-prose max-w-[74ch] text-sm ${tone}`}>
+    <div className={`thot-prose thot-activity-prose max-w-[74ch] text-sm ${tone}`}>
       <ReactMarkdown remarkPlugins={[remarkGfm]}>{formatModelActivity(entry.text)}</ReactMarkdown>
     </div>
   );
@@ -56,7 +56,13 @@ function ActivityRow({ entry }: { entry: ActivityEntry }) {
 }
 
 /** Left drawer showing the user-facing projection of the in-memory model activity timeline. */
-export function ModelActivityPanel({ onClose }: { onClose: () => void }) {
+export function ModelActivityPanel({
+  desktopSplit = false,
+  onClose,
+}: {
+  desktopSplit?: boolean;
+  onClose: () => void;
+}) {
   const activityLog = useSessionStore((s) => s.activityLog);
   const visibleActivity = activityLog.filter(isVisibleModelActivity);
   const visibleCount = visibleActivity.length;
@@ -70,7 +76,10 @@ export function ModelActivityPanel({ onClose }: { onClose: () => void }) {
   }, [visibleCount, visibleTail]);
 
   return (
-    <aside className="fixed inset-y-0 left-0 z-30 flex w-[min(90vw,24rem)] shrink-0 flex-col border-r border-border bg-sidebar lg:static lg:z-auto lg:w-[var(--activity-panel-width)] lg:border-r-0">
+    <aside className={desktopSplit
+      ? "static z-auto flex w-[var(--activity-panel-width)] shrink-0 flex-col border-r-0 bg-sidebar"
+      : "fixed inset-y-0 left-0 z-30 flex w-[min(90vw,24rem)] shrink-0 flex-col border-r border-border bg-sidebar"
+    }>
       <div className="flex items-center justify-between border-b border-border/60 px-3 py-2">
         <h2 className="font-heading text-sm font-semibold text-foreground">Model activity</h2>
         <Button size="icon-sm" variant="ghost" aria-label="Close model activity" onClick={onClose}>

frontend/src/shell/useActivityPanelResize.ts

@@ -71,11 +71,17 @@ export function resizeWidthFromKey(
   return null;
 }
 
-type DragState = { pointerId: number; startX: number; startWidth: number };
+type DragState = {
+  pointerId: number;
+  startX: number;
+  startWidth: number;
+  target: HTMLDivElement;
+};
 
 export function useActivityPanelResize(): {
   containerRef: RefObject<HTMLDivElement>;
   panelWidth: number;
+  desktopSplit: boolean;
   resizing: boolean;
   separatorProps: {
     role: "separator";
@@ -103,6 +109,7 @@ export function useActivityPanelResize(): {
   const dragRef = useRef<DragState | null>(null);
   const [panelWidth, setPanelWidth] = useState(initialWidthRef.current);
   const [bounds, setBounds] = useState(boundsRef.current);
+  const [desktopSplit, setDesktopSplit] = useState(false);
   const [resizing, setResizing] = useState(false);
 
   const applyWidth = useCallback((rawWidth: number, persist: boolean) => {
@@ -116,20 +123,42 @@ export function useActivityPanelResize(): {
   useLayoutEffect(() => {
     const container = containerRef.current;
     if (!container) return;
-    const syncBounds = () => {
+    const cancelDrag = () => {
-      if (container.clientWidth < ACTIVITY_PANEL_MIN_WIDTH + ACTIVITY_CENTRAL_MIN_WIDTH) return;
+      const drag = dragRef.current;
+      if (drag?.target.hasPointerCapture?.(drag.pointerId)) {
+        drag.target.releasePointerCapture?.(drag.pointerId);
+      }
+      dragRef.current = null;
+      setResizing(false);
+    };
+    const syncLayout = () => {
+      const viewportIsDesktop = window.matchMedia?.("(min-width: 1024px)").matches
+        ?? window.innerWidth >= 1024;
+      const hasUsableContainer = container.clientWidth >= ACTIVITY_PANEL_MIN_WIDTH + ACTIVITY_CENTRAL_MIN_WIDTH;
+      const nextDesktopSplit = viewportIsDesktop && hasUsableContainer;
+      setDesktopSplit(nextDesktopSplit);
+      if (!nextDesktopSplit) {
+        cancelDrag();
+        const savedWidth = loadActivityPanelWidth(storageRef.current);
+        widthRef.current = savedWidth;
+        setPanelWidth(savedWidth);
+        return;
+      }
       const nextBounds = getActivityPanelBounds(container.clientWidth);
       boundsRef.current = nextBounds;
       setBounds(nextBounds);
       applyWidth(widthRef.current, true);
     };
-    syncBounds();
+    syncLayout();
-    const observer = typeof ResizeObserver === "undefined" ? null : new ResizeObserver(syncBounds);
+    const observer = typeof ResizeObserver === "undefined" ? null : new ResizeObserver(syncLayout);
     observer?.observe(container);
-    window.addEventListener("resize", syncBounds);
+    const media = window.matchMedia?.("(min-width: 1024px)");
+    media?.addEventListener?.("change", syncLayout);
+    window.addEventListener("resize", syncLayout);
     return () => {
       observer?.disconnect();
-      window.removeEventListener("resize", syncBounds);
+      media?.removeEventListener?.("change", syncLayout);
+      window.removeEventListener("resize", syncLayout);
     };
   }, [applyWidth]);
 
@@ -138,6 +167,7 @@ export function useActivityPanelResize(): {
       pointerId: event.pointerId,
       startX: event.clientX,
       startWidth: widthRef.current,
+      target: event.currentTarget,
     };
     event.currentTarget.setPointerCapture?.(event.pointerId);
     setResizing(true);
@@ -174,6 +204,7 @@ export function useActivityPanelResize(): {
   return {
     containerRef,
     panelWidth,
+    desktopSplit,
     resizing,
     separatorProps: {
       role: "separator",