fix: capture the mouse on macOS while the cursor is on a client

On the server, while the cursor is on a client, every physical mouse move
warped the local cursor back to screen center and the tap callback returned
the mouse-move event instead of consuming it. That leaked continuous motion
to whatever local app sat under the centered cursor (e.g. a fullscreen game
or a centered video reacting to the phantom movement).

Capture the mouse the way macOS games do instead: dissociate the hardware
mouse from the cursor in leave() so the cursor freezes and hides, read raw
deltas from the event in onMouseMove(), consume off-screen moves in the tap
callback, and re-associate in enter() (disable() already does via
showCursor()). This retires the center-warp and the "bogus motion" edge
heuristic that only existed to paper over the warp.

fixes: #5871

src/lib/platform/OSXScreen.h

@@ -112,7 +112,7 @@ private:
   void sendClipboardEvent(EventTypes type, ClipboardID id) const;
 
   // message handlers
-  bool onMouseMove();
+  bool onMouseMove(CGEventRef event);
   // mouse button handler.  pressed is true if this is a mousedown
   // event, false if it is a mouseup event.  macButton is the index
   // of the button pressed using the mac button mapping.

src/lib/platform/OSXScreen.mm

@@ -769,6 +769,8 @@ void OSXScreen::enter()
   showCursor();
 
   if (m_isPrimary) {
+    // re-couple the mouse to the cursor, undoing the capture from leave()
+    CGAssociateMouseAndMouseCursorPosition(true);
     setZeroSuppressionInterval();
   } else {
     // reset buttons
@@ -798,8 +800,11 @@ void OSXScreen::leave()
 
   if (m_isPrimary) {
     avoidHesitatingCursor();
-    LOG_VERBOSE("centering cursor on leave: %+d, %+d", m_xCenter, m_yCenter);
-    warpCursor(m_xCenter, m_yCenter);
+
+    // capture the mouse: freeze the cursor so no local app sees motion while on
+    // a client (onMouseMove reads raw deltas instead). must follow hideCursor(),
+    // which re-associates. re-coupled in enter()/disable().
+    CGAssociateMouseAndMouseCursorPosition(false);
   }
 
   // now off screen
@@ -958,64 +963,40 @@ void OSXScreen::handleSystemEvent(const Event &event)
   }
 }
 
-bool OSXScreen::onMouseMove()
+bool OSXScreen::onMouseMove(CGEventRef event)
 {
-  // when we receive a mouse-move event, it is possible it was queued for a period
-  // and that the mouse has already moved again since then.  to handle this, we need
-  // to query the current mouse position rather than using the position in the event.
-  CGEventRef event = CGEventCreate(NULL);
-  CGPoint pos = CGEventGetLocation(event);
-  CFRelease(event);
-  CGFloat mx = pos.x;
-  CGFloat my = pos.y;
-
-  LOG_VERBOSE("mouse move %+f,%+f", mx, my);
-
-  CGFloat x = mx - m_xCursor;
-  CGFloat y = my - m_yCursor;
-
-  if ((x == 0 && y == 0) || (mx == m_xCenter && mx == m_yCenter)) {
-    return true;
-  }
-
-  // save position to compute delta of next motion
-  m_xCursor = (int32_t)mx;
-  m_yCursor = (int32_t)my;
-
   if (m_isOnScreen) {
-    // motion on primary screen
+    // motion on primary screen.  the event may have been queued a while, so
+    // query the live cursor position rather than the stale event position.
+    CGEventRef posEvent = CGEventCreate(NULL);
+    CGPoint pos = CGEventGetLocation(posEvent);
+    CFRelease(posEvent);
+    CGFloat mx = pos.x;
+    CGFloat my = pos.y;
+
+    LOG_VERBOSE("mouse move %+f,%+f", mx, my);
+
+    CGFloat x = mx - m_xCursor;
+    CGFloat y = my - m_yCursor;
+
+    if ((x == 0 && y == 0) || (mx == m_xCenter && mx == m_yCenter)) {
+      return true;
+    }
+
+    m_xCursor = (int32_t)mx;
+    m_yCursor = (int32_t)my;
+
     sendEvent(EventTypes::PrimaryScreenMotionOnPrimary, MotionInfo::alloc(m_xCursor, m_yCursor));
   } else {
-    // motion on secondary screen.  warp mouse back to
-    // center.
-    warpCursor(m_xCenter, m_yCenter);
+    // motion on secondary screen.  the cursor is frozen (see leave()), so read
+    // raw deltas from the event instead of diffing position.
+    int32_t dx = (int32_t)CGEventGetIntegerValueField(event, kCGMouseEventDeltaX);
+    int32_t dy = (int32_t)CGEventGetIntegerValueField(event, kCGMouseEventDeltaY);
 
-    // examine the motion.  if it's about the distance
-    // from the center of the screen to an edge then
-    // it's probably a bogus motion that we want to
-    // ignore (see warpCursorNoFlush() for a further
-    // description).
-    static int32_t bogusZoneSize = 10;
-    if (-x + bogusZoneSize > m_xCenter - m_x || x + bogusZoneSize > m_x + m_w - m_xCenter ||
-        -y + bogusZoneSize > m_yCenter - m_y || y + bogusZoneSize > m_y + m_h - m_yCenter) {
-      LOG_DEBUG("dropped bogus motion %+d,%+d", x, y);
-    } else {
-      // send motion
-      // Accumulate together the move into the running total
-      static CGFloat m_xFractionalMove = 0;
-      static CGFloat m_yFractionalMove = 0;
+    LOG_VERBOSE("mouse delta %+d,%+d", dx, dy);
 
-      m_xFractionalMove += x;
-      m_yFractionalMove += y;
-
-      // Return the integer part
-      int32_t intX = (int32_t)m_xFractionalMove;
-      int32_t intY = (int32_t)m_yFractionalMove;
-
-      // And keep only the fractional part
-      m_xFractionalMove -= intX;
-      m_yFractionalMove -= intY;
-      sendEvent(EventTypes::PrimaryScreenMotionOnSecondary, MotionInfo::alloc(intX, intY));
+    if (dx != 0 || dy != 0) {
+      sendEvent(EventTypes::PrimaryScreenMotionOnSecondary, MotionInfo::alloc(dx, dy));
     }
   }
 
@@ -1690,15 +1671,9 @@ CGEventRef OSXScreen::handleCGInputEvent(CGEventTapProxy proxy, CGEventType type
   case kCGEventRightMouseDragged:
   case kCGEventOtherMouseDragged:
   case kCGEventMouseMoved:
-    // we intentionally ignore the position in the event here as the events are
-    // queued and will no longer be accurate when we process them.
-    screen->onMouseMove();
-
-    // The system ignores our cursor-centering calls if
-    // we don't return the event. This should be harmless,
-    // but might register as slight movement to other apps
-    // on the system. It hasn't been a problem before, though.
-    return event;
+    // off-screen the cursor is frozen (see leave()), so fall through to consume
+    // the move below instead of returning (leaking) it to local apps.
+    screen->onMouseMove(event);
     break;
   case kCGEventScrollWheel:
     screen->onMouseWheel(