macos: hook up onDrop to move splits

macos/Sources/Features/Splits/TerminalSplitTreeView.swift

@@ -4,7 +4,7 @@ import os
 struct TerminalSplitTreeView: View {
     let tree: SplitTree<Ghostty.SurfaceView>
     let onResize: (SplitTree<Ghostty.SurfaceView>.Node, Double) -> Void
-    let onDrop: (Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
+    let onDrop: (_ source: Ghostty.SurfaceView, _ destination: Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
 
     var body: some View {
         if let node = tree.zoomed ?? tree.root {
@@ -28,7 +28,7 @@ struct TerminalSplitSubtreeView: View {
     let node: SplitTree<Ghostty.SurfaceView>.Node
     var isRoot: Bool = false
     let onResize: (SplitTree<Ghostty.SurfaceView>.Node, Double) -> Void
-    let onDrop: (Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
+    let onDrop: (_ source: Ghostty.SurfaceView, _ destination: Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
 
     var body: some View {
         switch (node) {
@@ -68,7 +68,7 @@ struct TerminalSplitSubtreeView: View {
 struct TerminalSplitLeaf: View {
     let surfaceView: Ghostty.SurfaceView
     let isSplit: Bool
-    let onDrop: (Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
+    let onDrop: (_ source: Ghostty.SurfaceView, _ destination: Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
     
     @State private var dropState: DropState = .idle
     
@@ -86,7 +86,8 @@ struct TerminalSplitLeaf: View {
                     .onDrop(of: [.ghosttySurfaceId], delegate: SplitDropDelegate(
                         dropState: $dropState,
                         viewSize: geometry.size,
-                        onDrop: { zone in onDrop(surfaceView, zone) }
+                        destinationSurface: surfaceView,
+                        onDrop: onDrop
                     ))
             }
         }
@@ -110,7 +111,8 @@ struct TerminalSplitLeaf: View {
     private struct SplitDropDelegate: DropDelegate {
         @Binding var dropState: DropState
         let viewSize: CGSize
-        let onDrop: (TerminalSplitDropZone) -> Void
+        let destinationSurface: Ghostty.SurfaceView
+        let onDrop: (_ source: Ghostty.SurfaceView, _ destination: Ghostty.SurfaceView, TerminalSplitDropZone) -> Void
         
         func validateDrop(info: DropInfo) -> Bool {
             info.hasItemsConforming(to: [.ghosttySurfaceId])
@@ -134,8 +136,27 @@ struct TerminalSplitLeaf: View {
         }
         
         func performDrop(info: DropInfo) -> Bool {
+            let zone = TerminalSplitDropZone.calculate(at: info.location, in: viewSize)
             dropState = .idle
-            onDrop(.calculate(at: info.location, in: viewSize))
+
+            // Load the dropped surface asynchronously using Transferable
+            let providers = info.itemProviders(for: [.ghosttySurfaceId])
+            guard let provider = providers.first else { return false }
+            _ = provider.loadTransferable(type: Ghostty.SurfaceView.self) { [weak destinationSurface] result in
+                switch result {
+                case .success(let sourceSurface):
+                    DispatchQueue.main.async {
+                        // Don't allow dropping on self
+                        guard let destinationSurface else { return }
+                        guard sourceSurface !== destinationSurface else { return }
+                        onDrop(sourceSurface, destinationSurface, zone)
+                    }
+                    
+                case .failure:
+                    break
+                }
+            }
+            
             return true
         }
     }

macos/Sources/Features/Terminal/BaseTerminalController.swift

@@ -827,6 +827,38 @@ class BaseTerminalController: NSWindowController,
         }
     }
 
+    func splitDidDrop(source: Ghostty.SurfaceView, destination: Ghostty.SurfaceView, zone: TerminalSplitDropZone) {
+        // Find the source node in the tree
+        guard let sourceNode = surfaceTree.root?.node(view: source) else {
+            Ghostty.logger.warning("source surface not found in tree during drop")
+            return
+        }
+
+        // Map drop zone to split direction
+        let direction: SplitTree<Ghostty.SurfaceView>.NewDirection = switch zone {
+        case .top: .up
+        case .bottom: .down
+        case .left: .left
+        case .right: .right
+        }
+
+        // Remove source from its current position first
+        let treeWithoutSource = surfaceTree.remove(sourceNode)
+
+        // Insert source at destination in the appropriate direction
+        do {
+            let newTree = try treeWithoutSource.insert(view: source, at: destination, direction: direction)
+            replaceSurfaceTree(
+                newTree,
+                moveFocusTo: source,
+                moveFocusFrom: focusedSurface,
+                undoAction: "Move Split")
+        } catch {
+            Ghostty.logger.warning("failed to insert surface during drop: \(error)")
+            return
+        }
+    }
+
     func performAction(_ action: String, on surfaceView: Ghostty.SurfaceView) {
         guard let surface = surfaceView.surface else { return }
         let len = action.utf8CString.count

macos/Sources/Features/Terminal/TerminalView.swift

@@ -20,6 +20,9 @@ protocol TerminalViewDelegate: AnyObject {
 
     /// A split is resizing to a given value.
     func splitDidResize(node: SplitTree<Ghostty.SurfaceView>.Node, to newRatio: Double)
+
+    /// A surface was dropped onto another surface to create a split.
+    func splitDidDrop(source: Ghostty.SurfaceView, destination: Ghostty.SurfaceView, zone: TerminalSplitDropZone)
 }
 
 /// The view model is a required implementation for TerminalView callers. This contains
@@ -83,8 +86,8 @@ struct TerminalView<ViewModel: TerminalViewModel>: View {
                     TerminalSplitTreeView(
                         tree: viewModel.surfaceTree,
                         onResize: { delegate?.splitDidResize(node: $0, to: $1) },
-                        onDrop: { surface, zone in
-                            Ghostty.logger.info("Drop on surface \(surface) in zone \(zone.rawValue)")
+                        onDrop: { source, destination, zone in
+                            delegate?.splitDidDrop(source: source, destination: destination, zone: zone)
                         })
                         .environmentObject(ghostty)
                         .focused($focused)