Fix title bar buttons on Windows

This fixes the broken mouse event handling by not consuming events
supposed to go to the title bar, making it possible to move/resize/close
the window.

vncviewer/Viewport.cxx

@@ -1170,6 +1170,9 @@ int Viewport::handleSystemPointerEvent(void *event, void *data)
       mouseButtonMask = 0;
       self->handlePointerEvent(lastPointerPos, 0);
     }
+#ifdef WIN32
+    mouseHasLeft = true;
+#endif
     return 0;
   }
 
@@ -1180,8 +1183,16 @@ int Viewport::handleSystemPointerEvent(void *event, void *data)
 
   mouse_x = GET_X_LPARAM(msg->lParam) - x();
   mouse_y = GET_Y_LPARAM(msg->lParam) - y();
-  if ((mouse_x < 0 || mouse_x > w() || mouse_y < 0 || mouse_y > h())
-      && !(mouseButtonMask & 0x187))
+
+  bool outOfBounds = (mouse_x < 0 || mouse_x > w() ||
+                      mouse_y < 0 || mouse_y > h());
+
+  // Leave events going to title bar
+  if (outOfBounds && mouseHasLeft)
+      return 0;
+
+  // Consume mouse events when dragging button outside window
+  if (outOfBounds && !(mouseButtonMask & 0x187))
       return 1;
 
   if (self->altGrArmed)