fix(canvas): persist SidePanel width to localStorage (issue #425)

Width was initialized to 480px on every render, so clicking a different
workspace node (which re-mounts SidePanel) discarded any resize the user
had done.

Fix:
- localStorage-backed useState initializer (SSR-safe typeof window guard)
- Validates the stored value: must be a finite integer ≥ 320px
- Persists the width in the mouseUp handler via a widthRef that stays in
  sync with the live drag value — avoids spamming localStorage on every
  pixel during the drag
- Extra guard: onMouseUp bails early if not actually dragging (prevents
  spurious saves on unrelated window mouseup events)
- Named constants replace magic numbers 480 / 320

Tests: 5 new cases in SidePanel.tabs.test.tsx — default fallback, valid
saved value, too-small saved value, NaN saved value, drag-persist roundtrip.

Closes #425

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canvas/src/components/SidePanel.tsx

@@ -19,6 +19,10 @@ import { ScheduleTab } from "./tabs/ScheduleTab";
 import { ChannelsTab } from "./tabs/ChannelsTab";
 import { summarizeWorkspaceCapabilities } from "@/store/canvas";
 
+const SIDEPANEL_WIDTH_KEY = "molecule:sidepanel-width";
+const SIDEPANEL_DEFAULT_WIDTH = 480;
+const SIDEPANEL_MIN_WIDTH = 320;
+
 const TABS: { id: PanelTab; label: string; icon: string }[] = [
   { id: "chat", label: "Chat", icon: "◈" },
   { id: "activity", label: "Activity", icon: "⊙" },
@@ -43,11 +47,19 @@ export function SidePanel() {
     s.nodes.find((n) => n.id === s.selectedNodeId)
   );
 
-  // Resizable panel width
-  const [width, setWidth] = useState(480);
+  // Resizable panel width — persisted across node selections via localStorage
+  const [width, setWidth] = useState<number>(() => {
+    if (typeof window === "undefined") return SIDEPANEL_DEFAULT_WIDTH;
+    const saved = localStorage.getItem(SIDEPANEL_WIDTH_KEY);
+    const parsed = saved ? parseInt(saved, 10) : NaN;
+    return Number.isFinite(parsed) && parsed >= SIDEPANEL_MIN_WIDTH
+      ? parsed
+      : SIDEPANEL_DEFAULT_WIDTH;
+  });
+  const widthRef = useRef(width); // tracks live drag value for the mouseup handler
   const dragging = useRef(false);
   const startX = useRef(0);
-  const startWidth = useRef(480);
+  const startWidth = useRef(SIDEPANEL_DEFAULT_WIDTH);
 
   const onMouseDown = useCallback((e: React.MouseEvent) => {
     e.preventDefault();
@@ -62,13 +74,20 @@ export function SidePanel() {
     const onMouseMove = (e: MouseEvent) => {
       if (!dragging.current) return;
       const delta = startX.current - e.clientX;
-      const newWidth = Math.min(Math.max(startWidth.current + delta, 320), window.innerWidth * 0.8);
+      const newWidth = Math.min(
+        Math.max(startWidth.current + delta, SIDEPANEL_MIN_WIDTH),
+        window.innerWidth * 0.8,
+      );
       setWidth(newWidth);
+      widthRef.current = newWidth; // keep ref in sync so mouseUp can persist it
     };
     const onMouseUp = () => {
+      if (!dragging.current) return;
       dragging.current = false;
       document.body.style.cursor = "";
       document.body.style.userSelect = "";
+      // Persist the final dragged width so it survives node re-selection
+      localStorage.setItem(SIDEPANEL_WIDTH_KEY, String(widthRef.current));
     };
     window.addEventListener("mousemove", onMouseMove);
     window.addEventListener("mouseup", onMouseUp);

canvas/src/components/__tests__/SidePanel.tabs.test.tsx

@@ -1,5 +1,5 @@
 // @vitest-environment jsdom
-import { describe, it, expect, vi, afterEach } from "vitest";
+import { describe, it, expect, vi, afterEach, beforeEach } from "vitest";
 import { render, screen, fireEvent, cleanup } from "@testing-library/react";
 
 afterEach(() => {
@@ -160,3 +160,59 @@ describe("SidePanel — ARIA tablist pattern", () => {
     expect(mockSetPanelTab).toHaveBeenCalledWith("events");
   });
 });
+
+describe("SidePanel — localStorage width persistence (issue #425)", () => {
+  const STORAGE_KEY = "molecule:sidepanel-width";
+
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  afterEach(() => {
+    cleanup();
+    localStorage.clear();
+  });
+
+  it("falls back to 480px when localStorage has no saved width", () => {
+    const { container } = render(<SidePanel />);
+    const panel = container.firstChild as HTMLElement;
+    // The outermost div has style={{ width }}
+    expect(panel.style.width).toBe("480px");
+  });
+
+  it("reads a valid saved width from localStorage on mount", () => {
+    localStorage.setItem(STORAGE_KEY, "600");
+    const { container } = render(<SidePanel />);
+    const panel = container.firstChild as HTMLElement;
+    expect(panel.style.width).toBe("600px");
+  });
+
+  it("falls back to 480px when localStorage value is below minimum (320px)", () => {
+    localStorage.setItem(STORAGE_KEY, "200");
+    const { container } = render(<SidePanel />);
+    const panel = container.firstChild as HTMLElement;
+    expect(panel.style.width).toBe("480px");
+  });
+
+  it("falls back to 480px when localStorage value is not a number", () => {
+    localStorage.setItem(STORAGE_KEY, "notanumber");
+    const { container } = render(<SidePanel />);
+    const panel = container.firstChild as HTMLElement;
+    expect(panel.style.width).toBe("480px");
+  });
+
+  it("persists width to localStorage on mouseup after drag", () => {
+    localStorage.setItem(STORAGE_KEY, "600");
+    render(<SidePanel />);
+    // Simulate a drag: mousedown on resize handle, mousemove, mouseup
+    fireEvent.mouseDown(document.querySelector(".cursor-col-resize")!, {
+      clientX: 100,
+    });
+    fireEvent.mouseMove(window, { clientX: 50 }); // dragged 50px left → wider
+    fireEvent.mouseUp(window);
+    // localStorage should have been updated to the new width
+    const saved = localStorage.getItem(STORAGE_KEY);
+    expect(saved).not.toBeNull();
+    expect(parseInt(saved!, 10)).toBeGreaterThanOrEqual(320);
+  });
+});