fix(desktop): restore workspace panel shrinking (#940)

Tested: cd desktop && bun run test -- src/pages/ActiveSession.test.tsx --run
Tested: cd desktop && bun run test -- src/stores/workspacePanelStore.test.ts --run
Tested: git diff --check
Not-tested: bun run check:desktop remains blocked by an unrelated MessageList timestamp assertion and existing TabBar unhandled rejections.
Confidence: high
Scope-risk: narrow

desktop/src/pages/ActiveSession.test.tsx

@@ -1441,6 +1441,34 @@ describe('ActiveSession task polling', () => {
     })
 
     expect(useWorkspacePanelStore.getState().width).toBe(WORKSPACE_PANEL_DEFAULT_WIDTH + 32)
+
+    vi.spyOn(workbenchPanel, 'getBoundingClientRect').mockReturnValue({
+      x: 0,
+      y: 0,
+      width: 558,
+      height: 720,
+      top: 0,
+      right: 558,
+      bottom: 720,
+      left: 0,
+      toJSON: () => ({}),
+    })
+
+    act(() => {
+      const pointerDown = createEvent.pointerDown(resizeHandle)
+      Object.defineProperty(pointerDown, 'button', { value: 0 })
+      Object.defineProperty(pointerDown, 'clientX', { value: 100 })
+      fireEvent(resizeHandle, pointerDown)
+    })
+
+    act(() => {
+      const pointerMove = new Event('pointermove')
+      Object.defineProperty(pointerMove, 'clientX', { value: 132 })
+      window.dispatchEvent(pointerMove)
+      window.dispatchEvent(new Event('pointerup'))
+    })
+
+    expect(useWorkspacePanelStore.getState().width).toBe(526)
   })
 
   it('does not render the workspace panel when closed or for member sessions', () => {

desktop/src/pages/ActiveSession.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useRef, useState } from 'react'
+import type { RefObject } from 'react'
 import { Target } from 'lucide-react'
 import {
   SCHEDULED_TAB_ID,
@@ -127,7 +128,14 @@ function ActiveGoalStrip({
   )
 }
 
-function WorkspaceResizeHandle() {
+function getRenderedWorkspacePanelWidth(panelRef: RefObject<HTMLElement>, fallbackWidth: number) {
+  const renderedWidth = panelRef.current?.getBoundingClientRect().width ?? 0
+  return Number.isFinite(renderedWidth) && renderedWidth > 0
+    ? renderedWidth
+    : fallbackWidth
+}
+
+function WorkspaceResizeHandle({ panelRef }: { panelRef: RefObject<HTMLElement> }) {
   const t = useTranslation()
   const width = useWorkspacePanelStore((state) => state.width)
   const setWidth = useWorkspacePanelStore((state) => state.setWidth)
@@ -177,16 +185,17 @@ function WorkspaceResizeHandle() {
       onPointerDown={(event) => {
         if (event.button !== 0) return
         event.preventDefault()
-        setDragState({ startX: event.clientX, startWidth: width })
+        setDragState({ startX: event.clientX, startWidth: getRenderedWorkspacePanelWidth(panelRef, width) })
       }}
       onKeyDown={(event) => {
+        const renderedWidth = getRenderedWorkspacePanelWidth(panelRef, width)
         if (event.key === 'ArrowLeft') {
           event.preventDefault()
-          setWidth(width + WORKSPACE_RESIZE_STEP)
+          setWidth(renderedWidth + WORKSPACE_RESIZE_STEP)
         }
         if (event.key === 'ArrowRight') {
           event.preventDefault()
-          setWidth(width - WORKSPACE_RESIZE_STEP)
+          setWidth(renderedWidth - WORKSPACE_RESIZE_STEP)
         }
       }}
       className="group relative z-10 flex w-2 shrink-0 cursor-col-resize items-stretch justify-center bg-[var(--color-surface)] outline-none focus-visible:bg-[var(--color-surface-container)]"
@@ -278,6 +287,7 @@ function TerminalResizeHandle() {
 
 export function ActiveSession() {
   const isMobileLayout = useMobileViewport() && !isDesktopRuntime()
+  const workbenchPanelRef = useRef<HTMLElement>(null)
   const activeTabId = useTabStore((s) => s.activeTabId)
   const [dismissedBackgroundTaskKeysBySession, setDismissedBackgroundTaskKeysBySession] = useState<Record<string, Set<string>>>({})
   const activeTabType = useTabStore((s) => s.tabs.find((tab) => tab.sessionId === s.activeTabId)?.type ?? null)
@@ -640,8 +650,9 @@ export function ActiveSession() {
 
         {showWorkbench ? (
           <>
-            <WorkspaceResizeHandle />
+            <WorkspaceResizeHandle panelRef={workbenchPanelRef} />
             <aside
+              ref={workbenchPanelRef}
               data-testid="workbench-panel"
               className="flex h-full shrink-0 flex-col border-l border-[var(--color-border)] bg-[var(--color-surface)]"
               style={{ width: rightPanelWidth, maxWidth: '62%', minWidth: 'min(420px, 54%)' }}

desktop/src/stores/workspacePanelStore.ts

@@ -8,7 +8,7 @@ import {
 } from '../api/sessions'
 
 export const WORKSPACE_PANEL_DEFAULT_WIDTH = 860
-export const WORKSPACE_PANEL_MIN_WIDTH = 640
+export const WORKSPACE_PANEL_MIN_WIDTH = 420
 export const WORKSPACE_PANEL_MAX_WIDTH = 1120
 
 export type WorkspacePanelView = 'changed' | 'all'