feat(ST-1116): mobile auto-switch tabs + back button in BacklogSplitPane

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

components/backlog/backlog-split-pane.tsx

@@ -0,0 +1,34 @@
+'use client'
+
+import { useState } from 'react'
+import { useSelectionStore } from '@/stores/selection-store'
+import { SplitPane, type SplitPaneProps } from '@/components/split-pane/split-pane'
+
+type Props = Omit<SplitPaneProps, 'activeTab' | 'onActiveTabChange'>
+
+export function BacklogSplitPane(props: Props) {
+  const { selectedPbiId, selectedStoryId } = useSelectionStore()
+  const [activeTab, setActiveTab] = useState(0)
+
+  // React-recommended "derived state from props" pattern: update state during render
+  // instead of useEffect to avoid cascading renders.
+  const [prevPbiId, setPrevPbiId] = useState(selectedPbiId)
+  const [prevStoryId, setPrevStoryId] = useState(selectedStoryId)
+
+  if (selectedStoryId !== prevStoryId) {
+    setPrevStoryId(selectedStoryId)
+    if (selectedStoryId) setActiveTab(2)
+  }
+  if (selectedPbiId !== prevPbiId) {
+    setPrevPbiId(selectedPbiId)
+    if (selectedPbiId && !selectedStoryId) setActiveTab(1)
+  }
+
+  return (
+    <SplitPane
+      {...props}
+      activeTab={activeTab}
+      onActiveTabChange={setActiveTab}
+    />
+  )
+}

components/split-pane/split-pane.tsx

@@ -39,6 +39,8 @@ export interface SplitPaneProps {
   tabLabels?: string[]       // mobile tab labels, defaults to "Pane N"
   minSize?: number           // minimum px per pane, default 200
   mobileBreakpoint?: number  // default 1024
+  activeTab?: number         // controlled: parent manages which tab is active
+  onActiveTabChange?: (index: number) => void
 }
 
 export function SplitPane({
@@ -48,7 +50,10 @@ export function SplitPane({
   tabLabels,
   minSize = 200,
   mobileBreakpoint = 1024,
+  activeTab: activeTabProp,
+  onActiveTabChange,
 }: SplitPaneProps) {
+  const isControlled = activeTabProp !== undefined
   const n = panes.length
   const containerRef = useRef<HTMLDivElement>(null)
   const splitsRef = useRef<number[]>(defaultSplit)
@@ -58,7 +63,13 @@ export function SplitPane({
   })
   const [dragging, setDragging] = useState<number | null>(null) // divider index (0..n-2)
   const [isMobile, setIsMobile] = useState(false)
-  const [activeTab, setActiveTab] = useState(0)
+  const [internalTab, setInternalTab] = useState(0)
+  const activeTab = isControlled ? activeTabProp : internalTab
+
+  const handleTabChange = (i: number) => {
+    if (!isControlled) setInternalTab(i)
+    onActiveTabChange?.(i)
+  }
 
   useEffect(() => { splitsRef.current = splits }, [splits])
 
@@ -113,11 +124,20 @@ export function SplitPane({
   if (isMobile) {
     return (
       <div className="flex flex-col h-full">
-        <div className="flex border-b border-border shrink-0">
+        <div className="flex items-center border-b border-border shrink-0">
+          {activeTab > 0 && (
+            <button
+              onClick={() => handleTabChange(activeTab - 1)}
+              className="px-3 py-2 text-sm text-muted-foreground hover:text-foreground transition-colors shrink-0"
+              aria-label="Terug"
+            >
+              ←
+            </button>
+          )}
           {panes.map((_, i) => (
             <button
               key={i}
-              onClick={() => setActiveTab(i)}
+              onClick={() => handleTabChange(i)}
               className={cn(
                 'flex-1 py-2 text-sm font-medium transition-colors',
                 activeTab === i