fix(client): share the side-card width across sessions (last drag wins)

The panel width lived in the per-session state (dsh-sidebar:v1:<id>), so a
drag in one conversation never carried to another: switching conversations
snapped the panel back to that session's own width. The width is a layout
preference, not session content, so it now persists under a cross-session
key (dsh-sidebar:v1:width):

- every persist writes the current width to the global key;
- loadState prefers the global width over a session's persisted one;
- a cache-hit session switch adopts the global width.

Backward compatible: with no global key present (first run / older data)
the behavior is unchanged. Adds a regression test covering drag-in-A ->
switch-to-B and drag-in-B -> switch-back-to-A.

src/client/state.ts

@@ -723,6 +723,44 @@ export function reconcileAgentTerminals(
 
 const STORAGE_PREFIX = 'dsh-sidebar:v1'
 
+/**
+ * Cross-session panel width: the last dragged width, shared by EVERY
+ * conversation (the panel width is a layout preference, not per-session
+ * content). Written on every persist, read at session load and on
+ * cache-hit session switches, so a drag in one conversation carries to all
+ * the others (last drag wins).
+ */
+const GLOBAL_WIDTH_KEY = 'dsh-sidebar:v1:width'
+
+/** Clamp one width to the contract and the current viewport (mirror of {@link setWidth}). */
+function clampWidth(width: number): number {
+  const max = typeof window !== 'undefined' ? Math.max(PANEL_MIN, window.innerWidth) : PANEL_MAX
+  return Math.min(max, Math.max(PANEL_MIN, Math.round(width)))
+}
+
+/** Read the cross-session panel width (undefined when never dragged). */
+function readGlobalWidth(): number | undefined {
+  try {
+    const raw = localStorage.getItem(GLOBAL_WIDTH_KEY)
+    if (raw !== null) {
+      const parsed = Number(raw)
+      if (Number.isFinite(parsed) && parsed > 0) return clampWidth(parsed)
+    }
+  } catch {
+    // Storage unavailable: fall back to the per-session behavior.
+  }
+  return undefined
+}
+
+/** Persist the cross-session panel width (best-effort, like the session states). */
+function writeGlobalWidth(width: number): void {
+  try {
+    localStorage.setItem(GLOBAL_WIDTH_KEY, String(width))
+  } catch {
+    // Storage full or unavailable: layout memory is best-effort.
+  }
+}
+
 /** Immutable snapshot handed to React (replaced only on real changes). */
 export interface SidebarSnapshot {
   sessionId: string | undefined
@@ -743,6 +781,10 @@ export function defaultWidthFor(viewport: number, percent: number): number {
 }
 
 function loadState(sessionId: string, prefs: SidebarPrefs): SidebarState {
+  // The panel width is a cross-session preference: the last dragged width
+  // wins over a session's own persisted value, so switching conversations
+  // keeps the width the user chose anywhere.
+  const globalWidth = readGlobalWidth()
   try {
     const raw = localStorage.getItem(`${STORAGE_PREFIX}:${sessionId}`)
     if (raw !== null) {
@@ -751,7 +793,9 @@ function loadState(sessionId: string, prefs: SidebarPrefs): SidebarState {
       // sanitize re-ids any duplicates the pre-seeding counter left behind.
       nextIdCounter = maxCounterId(parsed)
       const sanitized = sanitizeState(parsed)
-      if (sanitized !== undefined) return sanitized
+      if (sanitized !== undefined) {
+        return globalWidth === undefined ? sanitized : { ...sanitized, width: globalWidth }
+      }
     }
   } catch {
     // Corrupt or unavailable storage: fall through to the default.
@@ -767,9 +811,9 @@ function loadState(sessionId: string, prefs: SidebarPrefs): SidebarState {
   // first seeding is affected: once the user expands the drawer,
   // `panelOpen: true` persists like any other state.
   const viewport = typeof window !== 'undefined' ? window.innerWidth : undefined
-  const width = viewport === undefined
+  const width = globalWidth ?? (viewport === undefined
     ? PANEL_DEFAULT
-    : defaultWidthFor(viewport, prefs.defaultWidthPercent)
+    : defaultWidthFor(viewport, prefs.defaultWidthPercent))
   const openByDefault = prefs.openByDefault && (viewport === undefined || !isNarrowWidth(viewport))
   return makeDefaultState(width, openByDefault, prefs.tabsEnabled['explorer'] !== false)
 }
@@ -965,6 +1009,13 @@ export class SidebarStore {
         // counter below THIS session's persisted ids — re-seed so fresh
         // pane/split ids can never collide with its tree.
         nextIdCounter = maxCounterId(state)
+        // The panel width is cross-session: adopt the latest dragged width
+        // (a cached session keeps its own layout otherwise).
+        const globalWidth = readGlobalWidth()
+        if (globalWidth !== undefined && state.width !== globalWidth) {
+          state = { ...state, width: globalWidth }
+          this.bySession.set(sessionId, state)
+        }
       }
       this.snapshot = { sessionId, state, prefs: this.prefs }
     }
@@ -1021,6 +1072,9 @@ export class SidebarStore {
   }
 
   private schedulePersist(sessionId: string, state: SidebarState): void {
+    // Keep the cross-session width in sync: any width change (drag, fullscreen
+    // toggle) becomes the shared width for every conversation.
+    writeGlobalWidth(state.width)
     window.clearTimeout(this.persistTimer)
     this.persistTimer = window.setTimeout(() => {
       try {

tests/unit.spec.ts

@@ -4,7 +4,7 @@ import { parseLogLines, parsePorcelainZ } from '../src/git.ts'
 import { parseUnifiedDiff } from '../src/client/DiffView.tsx'
 import {
   activateTab, allLeaves, BOTTOM_DEFAULT, BOTTOM_MIN, closeTab, createSidebarStore, defaultWidthFor, insertLeafAt, makeDefaultState,
-  migrateBottomTabs, moveTab, moveTabToEdge, openDiffTab, openTabInActivePane, patchTab, reconcileAgentTerminals, resizeSplit, resizeSplitIn, sanitizeState, setBottomHeight, splitPane, tabOpenIn, toggleBottomPanel, toggleExpanded, togglePanel,
+  migrateBottomTabs, moveTab, moveTabToEdge, openDiffTab, openTabInActivePane, patchTab, reconcileAgentTerminals, resizeSplit, resizeSplitIn, sanitizeState, setBottomHeight, setWidth, splitPane, tabOpenIn, toggleBottomPanel, toggleExpanded, togglePanel,
   type SidebarState, type SidebarTab, type SplitNode,
 } from '../src/client/state.ts'
 import { loadPrefs, type SidebarSettingsClient } from '../src/client/prefs.ts'
@@ -1277,6 +1277,41 @@ describe('side card preferences', () => {
     expect(openStore.getSnapshot().state?.panelOpen).toBe(true)
   })
 
+  it('shares the panel width across sessions (last drag wins)', () => {
+    // A real localStorage stub so the cross-session width actually persists
+    // (the default no-op mock would silently drop the write).
+    const storage = new Map<string, string>()
+    const savedWindow = (globalThis as Record<string, unknown>).window
+    const savedStorage = (globalThis as Record<string, unknown>).localStorage
+    ;(globalThis as Record<string, unknown>).window = {
+      innerWidth: 1280,
+      clearTimeout: () => {},
+      setTimeout: () => 0,
+    }
+    ;(globalThis as Record<string, unknown>).localStorage = {
+      getItem: (k: string) => storage.get(k) ?? null,
+      setItem: (k: string, v: string) => { storage.set(k, String(v)) },
+      removeItem: (k: string) => { storage.delete(k) },
+      key: () => null,
+      get length() { return storage.size },
+    }
+    try {
+      const store = createSidebarStore()
+      store.setSession('A')
+      store.reduce(s => setWidth(s, 500))
+      // A fresh session adopts the width dragged in A, not its own default.
+      store.setSession('B')
+      expect(store.getSnapshot().state?.width).toBe(500)
+      // A later drag in B carries back to the cached session A.
+      store.reduce(s => setWidth(s, 600))
+      store.setSession('A')
+      expect(store.getSnapshot().state?.width).toBe(600)
+    } finally {
+      ;(globalThis as Record<string, unknown>).window = savedWindow
+      ;(globalThis as Record<string, unknown>).localStorage = savedStorage
+    }
+  })
+
   it('seeds a brand-new session COLLAPSED on narrow viewports (the panel is a full-screen drawer there)', () => {
     // Stub a narrow window (the file otherwise runs without one): only
     // innerWidth is read while seeding a fresh session.