'On start of layout' is 'On layout load complete'. Events can't run while it's busy loading, so such an event would be redundant, since the first thing that happens when it's ready is it triggers 'On start of layout'.
It might be possible to mitigate the problem with asynchronous texture loading, but WebGL doesn't support it yet AFAIK, plus it might be fairly difficult to work in to the engine (currently all existing projects assume no events fire during loading, so we'd have to be careful not to break anything).
There's the good old-fashioned workaround which people came up with in the Classic days: for one tick before changing layout, quickly bring up a loading screen. Then the previous layout is displaying "loading..." while the next layout loads up.