feat: add keyboard navigation for tab bar

src/App.tsx

@@ -271,6 +271,7 @@ function App() {
   const [resizeStartState, setResizeStartState] = useState<{ startX: number; startWidth: number } | null>(null);
   const [contextMenu, setContextMenu] = useState<{ x: number; y: number; selectedText: string } | null>(null);
   const [selectionRects, setSelectionRects] = useState<{ left: number; top: number; width: number; height: number }[]>([]);
+  const [tabFocusIndex, setTabFocusIndex] = useState(0);
 
   const contentRef = useRef<HTMLDivElement>(null);
   const sidebarRef = useRef<HTMLDivElement>(null);
@@ -279,6 +280,7 @@ function App() {
   const kineticDragActiveRef = useRef(false);
   const contextMenuRef = useRef<HTMLDivElement>(null);
   const tabScrollRef = useRef<HTMLDivElement>(null);
+  const tabRefs = useRef<(HTMLDivElement | null)[]>([]);
   const [tabScrollState, setTabScrollState] = useState({ canLeft: false, canRight: false });
   const tabsRef = useRef(tabs);
   tabsRef.current = tabs;
@@ -806,6 +808,40 @@ function App() {
     }
   };
 
+  // Tab bar keyboard navigation (WAI-ARIA tabs pattern with roving tabindex)
+  const handleTabListKeyDown = (e: React.KeyboardEvent) => {
+    if (e.key === 'ArrowRight') {
+      e.preventDefault();
+      const next = (tabFocusIndex + 1) % tabs.length;
+      setTabFocusIndex(next);
+      tabRefs.current[next]?.focus();
+    } else if (e.key === 'ArrowLeft') {
+      e.preventDefault();
+      const prev = (tabFocusIndex - 1 + tabs.length) % tabs.length;
+      setTabFocusIndex(prev);
+      tabRefs.current[prev]?.focus();
+    } else if (e.key === 'Home') {
+      e.preventDefault();
+      setTabFocusIndex(0);
+      tabRefs.current[0]?.focus();
+    } else if (e.key === 'End') {
+      e.preventDefault();
+      const last = tabs.length - 1;
+      setTabFocusIndex(last);
+      tabRefs.current[last]?.focus();
+    } else if (e.key === 'Delete') {
+      e.preventDefault();
+      if (tabs[tabFocusIndex]) closeTab(tabs[tabFocusIndex].id);
+    } else if (e.key === 'Enter' || e.key === ' ') {
+      e.preventDefault();
+      const tab = tabs[tabFocusIndex];
+      if (tab) {
+        setActiveTabId(tab.id);
+        parseHeadings(tab.content);
+      }
+    }
+  };
+
   // Auto-focus first menuitem when a dropdown opens
   useEffect(() => {
     if (menuOpen) {
@@ -817,6 +853,20 @@ function App() {
     }
   }, [menuOpen]);
 
+  // Clamp tabFocusIndex when tabs are closed
+  useEffect(() => {
+    if (tabFocusIndex >= tabs.length && tabs.length > 0) {
+      setTabFocusIndex(tabs.length - 1);
+    }
+  }, [tabs.length, tabFocusIndex]);
+
+  // Scroll the focused tab into view when tabFocusIndex changes
+  useEffect(() => {
+    if (tabs.length > 1) {
+      scrollToTabIndex(tabFocusIndex);
+    }
+  }, [tabFocusIndex, tabs.length, scrollToTabIndex]);
+
   const osScrollbarOptions = {
     scrollbars: { autoHide: 'scroll' as const, autoHideDelay: 800, theme: 'os-theme-dark' as const }
   };
@@ -901,10 +951,10 @@ function App() {
         {tabs.length > 1 && (
           <motion.div className="tab-bar" initial={{ height: 0, opacity: 0 }} animate={{ height: 38, opacity: 1 }} exit={{ height: 0, opacity: 0 }} transition={{ duration: 0.25, ease: materialEase }}>
             <button className={`tab-scroll-arrow ${tabScrollState.canLeft ? 'visible' : ''}`} aria-label="Scroll tabs left" onMouseDown={() => startScrollingTabs('left')} onMouseUp={stopScrollingTabs} onMouseLeave={stopScrollingTabs}><ChevronLeft size={14} /></button>
-            <div className="tab-scroll-container" ref={tabScrollRef} role="tablist">
+            <div className="tab-scroll-container" ref={tabScrollRef} role="tablist" onKeyDown={handleTabListKeyDown}>
               <AnimatePresence initial={false}>
-                {tabs.map(tab => (
-                  <motion.div layout key={tab.id} id={`tab-${tab.id}`} role="tab" aria-selected={tab.id === activeTabId} className={`tab ${tab.id === activeTabId ? 'active' : ''}`} initial={{ opacity: 0, scale: 0.9 }} animate={{ opacity: 1, scale: 1 }} exit={{ opacity: 0, scale: 0.9 }} transition={smoothTransition} onClick={() => { setActiveTabId(tab.id); parseHeadings(tab.content); }}>
+                {tabs.map((tab, index) => (
+                  <motion.div layout key={tab.id} id={`tab-${tab.id}`} role="tab" aria-selected={tab.id === activeTabId} tabIndex={index === tabFocusIndex ? 0 : -1} ref={(el) => { tabRefs.current[index] = el; }} className={`tab ${tab.id === activeTabId ? 'active' : ''}`} initial={{ opacity: 0, scale: 0.9 }} animate={{ opacity: 1, scale: 1 }} exit={{ opacity: 0, scale: 0.9 }} transition={smoothTransition} onClick={() => { setTabFocusIndex(index); setActiveTabId(tab.id); parseHeadings(tab.content); }}>
                     <span className="tab-title">{tab.title}</span>
                     <button className="tab-close" aria-label="Close tab" onClick={(e) => { e.stopPropagation(); closeTab(tab.id); }}><X size={12} /></button>
                   </motion.div>