Hi, we are developing a cross-platform library for creating desktop applications in C++: https://github.com/aseprite/laf For this reason, in macOS, we cannot rely on the default NSApplication.run() event loop, so we decided to implement our custom event loop using the nextEventMatchingMask method. Then, when a window is in fullscreen mode, for some reason the window stops receiving mouseMove events when the mouse pointer enters an area at the top of the window. You can see this issue in action by trying the following example project: https://github.com/martincapello/custom-event-loop-issue This project just opens one window and uses a custom event loop, it displays the current mouse position at every mouseMove event received, and when the aforementioned area is entered it suddenly stops updating. There is also a video showing how to reproduce it. I was able to see that when the position stops updating, we still receive mouseMove events, but for a different window, a borderless window that is added to the NSApplication.windows collection when switching to fullscreen, and which seems to be taken the mouseMove events before reaching the main window. Also, this issue doesn't happen when using the default NSApplication.run method, despite the borderless windows being added as well.