Prevent Joy-Con pointer jumps when surface tracking resumes

Sources/Switch2KitApp/Output/MouseController.swift

@@ -50,11 +50,14 @@ final class MouseController: @unchecked Sendable {
         defer { units[serial] = unit }
         let dx = Self.wrapDiff(state.mouseX, unit.lastX), dy = Self.wrapDiff(state.mouseY, unit.lastY)
         unit.lastX = state.mouseX; unit.lastY = state.mouseY
-        guard unit.primed else { unit.primed = true; return false }
         guard state.liftDistance != 0 && state.liftDistance < 1000 && state.surfaceQuality < 4000 else {
+            // Lost tracking breaks the counter baseline as well as fractional motion.
+            // Reacquisition may jump/wrap counters; its first usable report is not a delta.
+            unit.primed = false
             unit.residualX = 0; unit.residualY = 0
             return false
         }
+        guard unit.primed else { unit.primed = true; return false }
         let scale = 0.35 * configuration.mouseSensitivity
         let x = Double(dx) * scale + unit.residualX, y = Double(dy) * scale + unit.residualY
         let moveX = x.rounded(.towardZero), moveY = y.rounded(.towardZero)

tests/mouse/MouseTests.swift

@@ -53,6 +53,48 @@ final class CGEvent {
             state.mouseX -= 1; precondition(!input())
         }
         precondition(!mouse.handle(serial: "pro", model: .proController2, state: state, configuration: config))
+        surfaceReacquisition()
         print("PASS accepted pointer motion, noise, lift/quality, wrap, disabled/denied, edge clamping and event failure")
     }
+    static func surfaceReacquisition() {
+        for model in [Switch2.Model.joyCon2Left, .joyCon2Right] {
+            let mouse = MouseController()
+            mouse.updateContext(permission: true, screens: [])
+            var config = ControllerConfiguration(); config.mouseEnabled = true
+            var state = ControllerState()
+            func input() -> Bool {
+                mouse.handle(serial: "surface-test", model: model, state: state, configuration: config)
+            }
+            CGEvent.cursor = CGPoint(x: 100, y: 100)
+            state.mouseX = 1000; state.mouseY = 2000
+            // An initial no-surface report must not prime a usable counter baseline.
+            precondition(!input())
+            state.liftDistance = 10; state.mouseX = 40000; state.mouseY = 100
+            precondition(!input() && CGEvent.cursor == CGPoint(x: 100, y: 100),
+                         "First contact after lifted reports only establishes the optical baseline")
+            state.mouseX += 10
+            precondition(input() && CGEvent.cursor.x == 103)
+            for invalid in 0..<3 {
+                let before = CGEvent.cursor
+                state.liftDistance = invalid == 0 ? 0 : invalid == 1 ? 1000 : 10
+                state.surfaceQuality = invalid == 2 ? 4000 : 0
+                state.mouseX &+= 30000; state.mouseY &+= 30000
+                precondition(!input() && CGEvent.cursor == before)
+                // Counters may jump, including wrapping, while tracking was unusable.
+                state.liftDistance = 10; state.surfaceQuality = 0
+                state.mouseX = UInt16.max; state.mouseY = 500
+                precondition(!input() && CGEvent.cursor == before,
+                             "Reacquisition cannot turn an untracked interval into pointer movement")
+                state.mouseX = 3
+                precondition(input() && CGEvent.cursor.x == before.x + 1,
+                             "A following tracked report retains modulo counter movement")
+                // No fractional residue from before loss or the discarded interval survives.
+                state.mouseX += 1
+                precondition(!input())
+            }
+            mouse.reset()
+        }
+        print("PASS both Joy-Con halves re-prime after lift, out-of-range surface and low quality")
+    }
+
 }