Fix single-pane split-drag drop zones and default a center drop to split

apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx

@@ -92,6 +92,8 @@ export function SecondaryPanelLayout({
 }: SecondaryPanelLayoutProps) {
   const paneContext = useOptionalPaneContext();
   const secondaryPanelHost = paneContext?.secondaryPanelHost ?? null;
+  const inlinePaneTargetId =
+    secondaryPanelHost === null ? paneContext?.paneId : undefined;
   const renderAsDrawer = useIsCompactViewport();
   const sidebarDrawerShowing = useSyncExternalStore(
     subscribeCompactSidebarDrawerShowing,
@@ -297,6 +299,7 @@ export function SecondaryPanelLayout({
 
   const mainContent = (
     <div
+      data-split-pane-id={inlinePaneTargetId}
       data-conversation-collapsed={isMainCollapsed}
       inert={isMainCollapsed}
       className={cn(

apps/app/src/components/sidebar/usePaneContentSplitDrag.ts

@@ -127,6 +127,7 @@ export function usePaneContentSplitActions() {
             zone,
             threadAlreadyOpen: findPaneByContent(layout.root, content) !== null,
             atMaxPanes: countPanes(layout.root) >= MAX_PANES,
+            singlePane: countPanes(layout.root) === 1,
           });
         },
         onDrop: (target) => {

apps/app/src/components/sidebar/useThreadRowSplitDrag.ts

@@ -82,6 +82,7 @@ export function useThreadRowSplitDrag({
             threadAlreadyOpen:
               findPaneByThread(layout.root, projectId, threadId) !== null,
             atMaxPanes: countPanes(layout.root) >= MAX_PANES,
+            singlePane: countPanes(layout.root) === 1,
           });
         },
         onDrop: (target) => {

apps/app/src/lib/split-drag/zones.test.ts

@@ -89,6 +89,17 @@ describe("decideThreadDrop", () => {
     ).toEqual({ zone: "center", label: "Replace this chat" });
   });
 
+  it("splits a center zone when the workspace has a single pane", () => {
+    expect(
+      decideThreadDrop({
+        zone: "center",
+        threadAlreadyOpen: false,
+        atMaxPanes: false,
+        singlePane: true,
+      }),
+    ).toEqual({ zone: "right", label: "Split right" });
+  });
+
   it("coerces edges to center-replace at the pane cap", () => {
     expect(
       decideThreadDrop({

apps/app/src/lib/split-drag/zones.ts

@@ -85,22 +85,28 @@ interface ThreadDropInput {
   zone: SplitZone;
   threadAlreadyOpen: boolean;
   atMaxPanes: boolean;
+  singlePane?: boolean;
 }
 
 export function decideThreadDrop({
   zone,
   threadAlreadyOpen,
   atMaxPanes,
+  singlePane = false,
 }: ThreadDropInput): ZoneDecision {
   if (threadAlreadyOpen) {
     return { zone: "center", label: "Already open — focus pane" };
   }
   if (atMaxPanes) {
     return { zone: "center", label: "Replace this chat" };
   }
+  const resolvedZone = singlePane && zone === "center" ? "right" : zone;
   return {
-    zone,
-    label: zone === "center" ? "Replace this chat" : `Split ${zone}`,
+    zone: resolvedZone,
+    label:
+      resolvedZone === "center"
+        ? "Replace this chat"
+        : `Split ${resolvedZone}`,
   };
 }