This is for HTML5 builds (tested with Scale mode: Downscale Fit, but probably the same for other scaling modes)
On 1.2.163:
window.WINDOW_EVENT_RESIZED is being called immediately upon game launch (callback is registered from init)
On 1.2.164:
window.WINDOW_EVENT_RESIZED is only called if you start resizing the browser manually. However, the original game project’s dimensions may already be different upon game launch, so there is currently no way to detect the new dimensions immediately?