fix: add split pane button to tab bar for discoverability (#135)

* fix: add split pane button to tab bar for discoverability

The split pane feature was only accessible via right-click context menu
or Ctrl+\ keyboard shortcut. Add a subtle split icon (⧉) to the tab
bar that appears at low opacity in single-pane mode. Hidden in
multi-pane mode where pane headers already provide split/close controls.

* fix: address review — change tab-bar from tablist to toolbar role

The tab bar contains both tabs and action buttons (new workstream,
split pane), which is invalid for role=tablist. Change to role=toolbar
which correctly describes a container of mixed interactive controls.

* fix: address design review — WCAG contrast, ARIA structure, mobile

- Drop opacity approach, use border: dashed var(--border) matching
  #new-tab-btn pattern (fixes WCAG contrast failure at 35% opacity)
- Nest tabs in #tab-list[role=tablist] inside toolbar (fixes invalid
  role=tab children inside role=toolbar)
- Hide split button on mobile (<600px) where splits can't work
- Add aria-keyshortcuts to both action buttons

turnstone/ui/static/app.js

@@ -1112,6 +1112,19 @@ function updatePaneHeaders() {
   } else {
     root.classList.remove("multi-pane");
   }
+  // Hide tab-bar split button when already in multi-pane mode
+  var splitBtn = document.getElementById("split-btn");
+  if (splitBtn) {
+    if (leafCount > 1) {
+      splitBtn.classList.add("hidden");
+    } else {
+      splitBtn.classList.remove("hidden");
+    }
+  }
+}
+
+function splitFocusedPane() {
+  if (focusedPaneId) splitPane(focusedPaneId, "horizontal");
 }
 
 // --- Tree helpers ---
@@ -1860,10 +1873,11 @@ document
 // ===========================================================================
 
 var tabBar = document.getElementById("tab-bar");
+var tabList = document.getElementById("tab-list");
 var newTabBtn = document.getElementById("new-tab-btn");
 
 function renderTabBar() {
-  tabBar.querySelectorAll(".ws-tab").forEach(function (t) {
+  tabList.querySelectorAll(".ws-tab").forEach(function (t) {
     t.remove();
   });
 
@@ -1910,7 +1924,7 @@ function renderTabBar() {
       tab.appendChild(close);
     }
 
-    tabBar.insertBefore(tab, newTabBtn);
+    tabList.appendChild(tab);
   });
 }
 

turnstone/ui/static/index.html

@@ -35,8 +35,10 @@
   <button id="logout-btn" class="header-btn" onclick="logout()" style="display:none">logout</button>
 </div>
 
-<div id="tab-bar" role="tablist">
-  <button id="new-tab-btn" onclick="newWorkstream()" title="New workstream (Ctrl+T)" aria-label="New workstream">+</button>
+<div id="tab-bar" role="toolbar" aria-label="Workstreams">
+  <div id="tab-list" role="tablist"></div>
+  <button id="new-tab-btn" onclick="newWorkstream()" title="New workstream (Ctrl+T)" aria-label="New workstream" aria-keyshortcuts="Control+t">+</button>
+  <button id="split-btn" onclick="splitFocusedPane()" title="Split pane (Ctrl+\)" aria-label="Split pane" aria-keyshortcuts="Control+Backslash">&#x29C9;</button>
 </div>
 
 <div id="dashboard" class="dashboard-overlay" role="dialog" aria-modal="true" aria-label="Dashboard">

turnstone/ui/static/style.css

@@ -112,6 +112,7 @@
   #hamburger-btn { width: 40px; height: 40px; }
   .hmenu-item { padding: 12px 14px; }
   .ws-tab .tab-close { opacity: 1; padding: 4px 6px; font-size: 16px; }
+  #split-btn { display: none; }
 }
 
 /* ==========================================================================
@@ -127,6 +128,11 @@
   flex-shrink: 0;
   overflow-x: auto;
 }
+#tab-list {
+  display: flex;
+  align-items: center;
+  gap: 2px;
+}
 #tab-bar::-webkit-scrollbar { height: 3px; }
 #tab-bar::-webkit-scrollbar-thumb { background: var(--border-strong); border-radius: 2px; }
 
@@ -190,6 +196,22 @@
 }
 #new-tab-btn:hover { background: var(--bg-highlight); color: var(--accent); border-color: var(--accent); }
 
+#split-btn {
+  background: none;
+  border: 1px dashed var(--border);
+  color: var(--fg-dim);
+  border-radius: var(--radius-sm) var(--radius-sm) 0 0;
+  padding: 6px 8px;
+  cursor: pointer;
+  font-family: inherit;
+  font-size: 13px;
+  line-height: 1;
+  transition: color 0.15s, border-color 0.15s, background 0.15s;
+  margin-left: 2px;
+}
+#split-btn:hover { background: var(--bg-highlight); color: var(--accent); border-color: var(--accent); }
+#split-btn.hidden { display: none; }
+
 /* ==========================================================================
    Split panes
    ========================================================================== */
@@ -1259,7 +1281,7 @@ body { position: static; }
   .tool-output-stream { animation: none; border-left-color: var(--accent); }
   .judge-spinner-dot { animation: none; opacity: 1; }
   .thinking-indicator::after { animation: none; content: '...'; }
-  .ws-tab, .ws-tab .tab-close, #new-tab-btn,
+  .ws-tab, .ws-tab .tab-close, #new-tab-btn, #split-btn,
   .hmenu-item, .dashboard-card,
   .approval-btn, .approval-feedback-input,
   #plan-buttons button, .pane-input-area button,