diff --git a/frontend/src/components/SessionView.tsx b/frontend/src/components/SessionView.tsx
index a6f7afb2..43ccce4c 100644
--- a/frontend/src/components/SessionView.tsx
+++ b/frontend/src/components/SessionView.tsx
@@ -36,10 +36,12 @@ import {
   findGroup,
   primaryGroup,
   allGroups,
+  allPanelIds,
   findGroupInDirection,
   updateSizes,
   findGroupContainingPanel,
   subsetInsertIndex,
+  mergeAllGroups,
   type DropZone,
 } from '../utils/panelLayout';
 import { Download, Upload, GitMerge, GitPullRequestArrow, Terminal, ChevronDown, ChevronUp, RefreshCw, Archive, ArchiveRestore, GitCommitHorizontal, TerminalSquare, Undo2, X } from 'lucide-react';
@@ -263,10 +265,10 @@ export const SessionView = memo(() => {
         const pinned = loadedPanels.find(p => p.type === 'terminal');
         const livePanels = pinned ? loadedPanels.filter(p => p.id !== pinned.id) : loadedPanels;
 
-        // Sort for initial layout creation (diff first, explorer second, then position)
+        // Sort for initial layout creation (explorer first, diff second, then position)
         const typeOrder = (type: string) => {
-          if (type === 'diff') return 0;
-          if (type === 'explorer') return 1;
+          if (type === 'explorer') return 0;
+          if (type === 'diff') return 1;
           return 2;
         };
         const sortedLive = [...livePanels].sort((a, b) => {
@@ -415,11 +417,11 @@ export const SessionView = memo(() => {
     [sessionPanels, defaultTerminalPanel]
   );
 
-  // Sort tab bar panels same as PanelTabBar: diff first, explorer second, then by position
+  // Sort tab bar panels same as PanelTabBar: explorer first, diff second, then by position
   const sortedSessionPanels = useMemo(() => {
     const typeOrder = (type: string) => {
-      if (type === 'diff') return 0;
-      if (type === 'explorer') return 1;
+      if (type === 'explorer') return 0;
+      if (type === 'diff') return 1;
       return 2;
     };
     return [...tabBarPanels].sort((a, b) => {
@@ -464,13 +466,18 @@ export const SessionView = memo(() => {
     return primaryGroupNode.panelIds.map(id => panelMap.get(id)).filter((p): p is ToolPanel => !!p);
   }, [primaryGroupNode, tabBarPanels]);
   const isSplitLayout = sessionLayout?.root.type === 'split';
-  /** What the top bar shows: everything when unsplit; only the primary
-      group's permanent tabs once split (working tabs live in group strips). */
+  /** What the top bar shows: everything when unsplit; once split, the
+      permanent tool tabs hoisted from EVERY group (in reading order), so the
+      defaults stay pinned to the top bar no matter which group owns them.
+      Clicking one routes to its owning group via handlePanelSelect. */
   const topBarPanels = useMemo(() => {
     if (!primaryGroupPanels) return undefined;
-    if (!isSplitLayout) return primaryGroupPanels;
-    return primaryGroupPanels.filter(p => p.metadata?.permanent === true);
-  }, [primaryGroupPanels, isSplitLayout]);
+    if (!isSplitLayout || !sessionLayout) return primaryGroupPanels;
+    const panelMap = new Map(tabBarPanels.map(p => [p.id, p]));
+    return allPanelIds(sessionLayout.root)
+      .map(id => panelMap.get(id))
+      .filter((p): p is ToolPanel => !!p && p.metadata?.permanent === true);
+  }, [primaryGroupPanels, isSplitLayout, sessionLayout, tabBarPanels]);
 
   const getPanelTabPresentation = useCallback<PanelTabPresentationResolver>((panel) => {
     if (panel.type !== 'diff') return undefined;
@@ -1101,17 +1108,33 @@ export const SessionView = memo(() => {
     setDropZones(new Map());
   }, [activeSession, applyLayout, preferWorkingActive]);
 
-  // Stable identity for the primary strip's drop handler so memo(PanelTabBar)
-  // holds. When split, the top bar shows only the permanent subset, so its
-  // drop indexes must translate to the group's full panel order.
+  // Top-bar drops are the un-split gesture: merge every group back into the
+  // primary group and place the dropped tab at the indicated position. When
+  // the pane isn't split, the merge is the identity and this is a plain
+  // reorder. The top bar shows only the permanent subset while split, so its
+  // drop indexes translate to the group's full panel order first.
   const primaryGroupId = primaryGroupNode?.id;
   const handlePrimaryStripDrop = useCallback((panelId: string, insertIndex: number) => {
-    if (!primaryGroupId || !primaryGroupNode) return;
+    if (!primaryGroupId || !primaryGroupNode || !activeSession) return;
+    const sid = activeSession.id;
+    const currentLayout = usePanelStore.getState().layouts[sid];
+    if (!currentLayout) return;
+    const merged = mergeAllGroups(currentLayout.root);
+    // The top bar shows the hoisted permanent subset while split; translate
+    // its drop index against the merged (post-un-split) panel order.
     const fullIndex = isSplitLayout && topBarPanels
-      ? subsetInsertIndex(primaryGroupNode.panelIds, topBarPanels.map(p => p.id), insertIndex)
+      ? subsetInsertIndex(merged.panelIds, topBarPanels.map(p => p.id), insertIndex)
       : insertIndex;
-    handleStripDrop(primaryGroupId, panelId, fullIndex);
-  }, [primaryGroupId, primaryGroupNode, isSplitLayout, topBarPanels, handleStripDrop]);
+    const newRoot = movePanelInLayout(merged, panelId, { groupId: merged.id, index: fullIndex });
+    applyLayout(sid, {
+      ...currentLayout,
+      root: newRoot,
+      focusedGroupId: merged.id,
+      zoomedGroupId: null,
+    });
+    setDraggedPanelId(null);
+    setDropZones(new Map());
+  }, [primaryGroupId, primaryGroupNode, activeSession, isSplitLayout, topBarPanels, applyLayout]);
 
   // --- Editor stage element (shared by both layouts) ---
   const editorStageElement = useMemo(() => {
@@ -1707,7 +1730,7 @@ export const SessionView = memo(() => {
           onToggleDetailPanel={handleToggleDetailPanel}
           detailPanelVisible={detailVisible}
           primaryGroupPanels={topBarPanels}
-          primaryGroupActivePanelId={primaryGroupNode?.activePanelId}
+          primaryGroupActivePanelId={isSplitLayout ? (currentActivePanel?.id ?? null) : primaryGroupNode?.activePanelId}
           primaryGroupFocused={!primaryGroupNode || primaryGroupNode.id === focusedGroupId}
           tabsInGroups={isSplitLayout}
           onDragStart={handleDragStart}
diff --git a/frontend/src/components/panels/PanelGroupView.tsx b/frontend/src/components/panels/PanelGroupView.tsx
index 944a1594..e39e7aa1 100644
--- a/frontend/src/components/panels/PanelGroupView.tsx
+++ b/frontend/src/components/panels/PanelGroupView.tsx
@@ -157,13 +157,13 @@ export const PanelGroupView: React.FC<PanelGroupViewProps> = React.memo(({
     return group.panelIds.map(id => panelMap.get(id)).filter((p): p is ToolPanel => !!p);
   }, [group.panelIds, groupPanels]);
 
-  // The primary group's permanent tabs (Diff/Explorer/Browser) stay up in
-  // PanelTabBar; its strip carries only the working tabs. Secondary groups
-  // show their full membership (a permanent tab dragged into one lives there).
-  const stripPanels = useMemo(() => {
-    if (!isPrimary) return orderedPanels;
-    return orderedPanels.filter(p => p.metadata?.permanent !== true);
-  }, [isPrimary, orderedPanels]);
+  // Permanent tool tabs (Diff/Explorer/Browser) are hoisted to PanelTabBar
+  // from EVERY group while split, so strips carry only working tabs. Their
+  // content still renders inside whichever group owns them.
+  const stripPanels = useMemo(
+    () => orderedPanels.filter(p => p.metadata?.permanent !== true),
+    [orderedPanels],
+  );
 
   // Strip drop indexes are relative to the displayed subset; translate to the
   // group's full panel order before moving.
@@ -198,6 +198,7 @@ export const PanelGroupView: React.FC<PanelGroupViewProps> = React.memo(({
             activePanelId={group.activePanelId}
             onPanelSelect={onPanelSelect}
             onPanelClose={onPanelClose}
+            isPrimary={isPrimary}
             isFocused={isFocusedGroup}
             variant="compact"
             onDragStart={onDragStart}
diff --git a/frontend/src/components/panels/PanelTabBar.tsx b/frontend/src/components/panels/PanelTabBar.tsx
index e76823e4..56590eec 100644
--- a/frontend/src/components/panels/PanelTabBar.tsx
+++ b/frontend/src/components/panels/PanelTabBar.tsx
@@ -458,11 +458,17 @@ export const PanelTabBar: React.FC<PanelTabBarProps> = memo(({
     }
   };
 
-  // Sort panels: diff first, explorer second, then by position
+  // Whether a tab drag is hovering the bar (drives the un-split affordance)
+  const [dragOverBar, setDragOverBar] = useState(false);
+  useEffect(() => {
+    if (!isTabDragging) setDragOverBar(false);
+  }, [isTabDragging]);
+
+  // Sort panels: explorer first, diff second, then by position
   const sortedPanels = useMemo(() => {
     const typeOrder = (type: string) => {
-      if (type === 'diff') return 0;
-      if (type === 'explorer') return 1;
+      if (type === 'explorer') return 0;
+      if (type === 'diff') return 1;
       if (type === 'browser') return 2;
       return 3;
     };
@@ -478,10 +484,28 @@ export const PanelTabBar: React.FC<PanelTabBarProps> = memo(({
     <div className="panel-tab-bar bg-bg-chrome flex-shrink-0">
       {/* Flex container */}
       <div
-        className="flex items-center min-h-[var(--panel-tab-height)] px-2"
+        className="relative flex items-center min-h-[var(--panel-tab-height)] px-2"
         role="tablist"
         aria-label="Panel Tabs"
+        onDragOver={tabsInGroups && isTabDragging ? () => setDragOverBar(true) : undefined}
+        onDragLeave={tabsInGroups && isTabDragging ? () => setDragOverBar(false) : undefined}
       >
+        {/* Un-split affordance: dropping a tab on the top bar while split
+            merges every group back into the primary group. Advertise that
+            while a drag hovers the bar (pointer-events-none so drops pass
+            through to the strip). */}
+        {tabsInGroups && isTabDragging && dragOverBar && (
+          <div className="pointer-events-none absolute inset-0 z-10 flex items-center justify-center">
+            <span className="inline-flex items-center gap-1.5 px-2.5 py-1 rounded-full text-[11px] bg-surface-primary border border-[color-mix(in_srgb,var(--color-interactive-primary)_40%,transparent)] text-text-secondary shadow-dropdown">
+              <svg width="12" height="12" viewBox="0 0 16 16" fill="none" stroke="currentColor" strokeWidth="1.5" aria-hidden="true">
+                <rect x="1.5" y="2.5" width="13" height="11" rx="1.5" />
+                <line x1="8" y1="2.5" x2="8" y2="13.5" strokeDasharray="2 2" opacity="0.5" />
+                <path d="M5.5 8h5M9 6.5 10.5 8 9 9.5" />
+              </svg>
+              Drop to merge all tabs back here
+            </span>
+          </div>
+        )}
         {/* Scrollable tab area — delegated to PanelTabStrip. When the pane is
             split, SessionView passes only the primary group's permanent tabs
             here (working tabs live in the group strips); shortcut hints are
diff --git a/frontend/src/components/panels/PanelTabStrip.tsx b/frontend/src/components/panels/PanelTabStrip.tsx
index 23ecdfc4..2c9d3218 100644
--- a/frontend/src/components/panels/PanelTabStrip.tsx
+++ b/frontend/src/components/panels/PanelTabStrip.tsx
@@ -222,6 +222,18 @@ export const PanelTabStrip: React.FC<PanelTabStripProps> = React.memo(({
     if (el) el.focus();
   }, []);
 
+  // The divider separates the default tool tabs (diff/explorer/browser) from
+  // the working tabs. It sits after the RIGHTMOST default in the strip, not
+  // after a hardcoded type, so reordering the defaults moves it correctly.
+  // Suppressed when the rightmost default is the strip's last tab.
+  const lastDefaultIndex = useMemo(() => {
+    let last = -1;
+    panels.forEach((p, idx) => {
+      if (p.type === 'diff' || p.type === 'explorer' || p.type === 'browser') last = idx;
+    });
+    return last;
+  }, [panels]);
+
   // Shortcut hints are only truthful when the hotkeys actually target this
   // strip's panels: Mod+Shift+1-9 act on the focused group, so hide hints
   // while a different group has focus.
@@ -339,7 +351,9 @@ export const PanelTabStrip: React.FC<PanelTabStripProps> = React.memo(({
                   )} />
                 )}
                 {getPanelIcon(panel.type, panel, compact ? 'w-3.5 h-3.5' : 'w-4 h-4')}
-                <span>{displayTitle}</span>
+                {/* Bold marks the primary group's strip: the group the top
+                    bar's tool tabs and the un-split gesture belong to */}
+                <span className={cn(compact && isPrimary && "font-semibold")}>{displayTitle}</span>
               </span>
             )}
 
@@ -357,7 +371,7 @@ export const PanelTabStrip: React.FC<PanelTabStripProps> = React.memo(({
           </div>
         );
 
-        const showDividerAfter = !compact && panel.type === 'browser';
+        const showDividerAfter = !compact && index === lastDefaultIndex && index < panels.length - 1;
         const divider = showDividerAfter ? (
           <div className="h-4 w-px bg-border-primary mx-1 flex-shrink-0" aria-hidden="true" />
         ) : null;
diff --git a/frontend/src/utils/panelLayout.test.ts b/frontend/src/utils/panelLayout.test.ts
index 18b5ed1e..49009fcf 100644
--- a/frontend/src/utils/panelLayout.test.ts
+++ b/frontend/src/utils/panelLayout.test.ts
@@ -30,6 +30,7 @@ import {
   findGroupInDirection,
   dropZoneFor,
   subsetInsertIndex,
+  mergeAllGroups,
 } from './panelLayout';
 
 // ---------------------------------------------------------------------------
@@ -430,6 +431,34 @@ describe('directional focus geometry', () => {
   });
 });
 
+// ---------------------------------------------------------------------------
+// mergeAllGroups
+// ---------------------------------------------------------------------------
+
+describe('mergeAllGroups', () => {
+  it('returns a single group unchanged', () => {
+    const g = group('g1', ['a', 'b'], 'b');
+    expect(mergeAllGroups(g)).toBe(g);
+  });
+
+  it('collapses a nested tree into the primary group, preserving id, order, and active', () => {
+    const tree = split('s1', 'row', [
+      group('g1', ['a', 'b'], 'b'),
+      split('s2', 'column', [group('g2', ['c']), group('g3', ['d'])]),
+    ]);
+    const merged = mergeAllGroups(tree);
+    expect(merged.id).toBe('g1');
+    expect(merged.panelIds).toEqual(['a', 'b', 'c', 'd']);
+    expect(merged.activePanelId).toBe('b');
+  });
+
+  it('dedupes panel ids, keeping the first occurrence', () => {
+    const tree = split('s1', 'row', [group('g1', ['a', 'b']), group('g2', ['a', 'c'])]);
+    const merged = mergeAllGroups(tree);
+    expect(merged.panelIds).toEqual(['a', 'b', 'c']);
+  });
+});
+
 // ---------------------------------------------------------------------------
 // subsetInsertIndex
 // ---------------------------------------------------------------------------
diff --git a/frontend/src/utils/panelLayout.ts b/frontend/src/utils/panelLayout.ts
index 9f7b3a20..bf22adb8 100644
--- a/frontend/src/utils/panelLayout.ts
+++ b/frontend/src/utils/panelLayout.ts
@@ -529,6 +529,34 @@ export function reconcile(
   };
 }
 
+// ---------------------------------------------------------------------------
+// Merge all groups (the un-split gesture)
+// ---------------------------------------------------------------------------
+
+/**
+ * Collapse the entire tree into the primary group. Panel order: the primary
+ * group's panels first, then the remaining groups' panels in reading order.
+ * The primary group's id and active tab survive.
+ */
+export function mergeAllGroups(root: PanelLayoutNode): PanelGroupNode {
+  if (root.type === 'group') return root;
+  const primary = primaryGroup(root);
+  const seen = new Set<string>();
+  const ids: string[] = [];
+  for (const id of allPanelIds(root)) {
+    if (!seen.has(id)) {
+      seen.add(id);
+      ids.push(id);
+    }
+  }
+  return {
+    type: 'group',
+    id: primary.id,
+    panelIds: ids,
+    activePanelId: primary.activePanelId ?? ids[0] ?? null,
+  };
+}
+
 // ---------------------------------------------------------------------------
 // Subset index translation
 // ---------------------------------------------------------------------------
diff --git a/main/src/services/sessionManager.ts b/main/src/services/sessionManager.ts
index 5fd01d18..ec663ce5 100644
--- a/main/src/services/sessionManager.ts
+++ b/main/src/services/sessionManager.ts
@@ -474,6 +474,14 @@ export class SessionManager extends EventEmitter {
       
       await panelManager.ensureExplorerPanel(session.id);
       await panelManager.ensureDiffPanel(session.id);
+      // Explorer is the intended initial tab (the diff creation above just
+      // stole active by being the last createPanel).
+      const explorerPanel = panelManager
+        .getPanelsForSession(session.id)
+        .find((p) => p.type === 'explorer');
+      if (explorerPanel) {
+        await panelManager.setActivePanel(session.id, explorerPanel.id);
+      }
       return session;
     });
   }
diff --git a/main/src/services/taskQueue.ts b/main/src/services/taskQueue.ts
index bbdab199..dfc9c2ed 100644
--- a/main/src/services/taskQueue.ts
+++ b/main/src/services/taskQueue.ts
@@ -274,6 +274,16 @@ export class TaskQueue {
           panelManager.ensureBrowserPanel(session.id),
         ]);
 
+        // Each createPanel marks the new panel active, so the parallel ensures
+        // leave a race winner focused. Explorer is the intended initial tab
+        // (renders instantly; diff can take a moment on big worktrees).
+        const explorerPanel = panelManager
+          .getPanelsForSession(session.id)
+          .find((p) => p.type === 'explorer');
+        if (explorerPanel) {
+          await panelManager.setActivePanel(session.id, explorerPanel.id);
+        }
+
         // Emit the session-created event BEFORE running build script so UI shows immediately
         sessionManager.emitSessionCreated(session);