cmux-next: debug.mouse middle clicks keep their window-local point

Packages/macOS/CmuxNext/Sources/CmuxNextApp/InputVerification/DebugMouse.swift

@@ -131,11 +131,22 @@ enum DebugMouse {
         ) else { return nil }
         guard [.otherMouseDown, .otherMouseUp, .otherMouseDragged].contains(type) else { return event }
         // NSEvent.mouseEvent leaves buttonNumber 0 on otherMouse events; the
-        // middle button is 2, which the views that handle it check. The
-        // copy keeps the window and the window-local point (on a Mac with a
-        // display; DebugMouseButtonTests).
+        // middle button is 2, which the views that handle it check. AppKit
+        // maps the copy back through the window server's frame for the window,
+        // which for a window not on screen is not the window's own frame, so the
+        // copy is moved by what that mapping got wrong (DebugMouseButtonTests).
         guard let cg = event.cgEvent?.copy() else { return nil }
         cg.setIntegerValueField(.mouseEventButtonNumber, value: 2)
+        return keepingWindowPoint(cg, event.locationInWindow)
+    }
+
+    /// `cg` as an NSEvent at window-local `base`: built once, then moved by the
+    /// difference between `base` and the point AppKit mapped (global y grows down).
+    static func keepingWindowPoint(_ cg: CGEvent, _ base: NSPoint) -> NSEvent? {
+        guard let first = NSEvent(cgEvent: cg) else { return nil }
+        let miss = NSPoint(x: base.x - first.locationInWindow.x, y: base.y - first.locationInWindow.y)
+        guard miss != .zero else { return first }
+        cg.location = CGPoint(x: cg.location.x + miss.x, y: cg.location.y - miss.y)
         return NSEvent(cgEvent: cg)
     }
 

Packages/macOS/CmuxNext/Tests/CmuxNextSidebarTests/WorkspaceRowMiddleClickTests.swift

@@ -71,9 +71,14 @@ import Testing
                 with: type, location: at, modifierFlags: [], timestamp: ProcessInfo.processInfo.systemUptime,
                 windowNumber: h.window.windowNumber, context: nil, eventNumber: 0, clickCount: 1,
                 pressure: type == .otherMouseUp ? 0 : 1))
-            // As debug.mouse makes them: NSEvent.mouseEvent leaves buttonNumber 0, the middle button is 2.
+            // As debug.mouse makes them (DebugMouse.keepingWindowPoint): NSEvent.mouseEvent leaves
+            // buttonNumber 0, the middle button is 2, and the copy is moved by what AppKit's mapping
+            // through the window server's frame (not this unshown window's) got wrong.
             let cg = try #require(made.cgEvent?.copy())
             cg.setIntegerValueField(.mouseEventButtonNumber, value: 2)
+            let first = try #require(NSEvent(cgEvent: cg))
+            cg.location = CGPoint(x: cg.location.x + at.x - first.locationInWindow.x,
+                                  y: cg.location.y - (at.y - first.locationInWindow.y))
             let event = try #require(NSEvent(cgEvent: cg))
             #expect(event.buttonNumber == 2)
             #expect(event.locationInWindow == at)