Periodic flashing in Safari (only)

This SDL2-based [Newton's Cradle program](https://wasm.bbcbasic.co.uk/bbcsdl.html?run=examples/physics/%28cradle%29.bbc) was compiled with the latest version of Emscripten (3.1.31). When running in Safari, only, it suffers from 'flashing' as if the display is periodically being blanked for one field period - at least it does on my 'Apple Silicon' Mac Mini.

I don't see this effect in Chrome-based browsers or Firefox (whether desktop or Android); it's a multithreaded app so won't run at all in iOS. Even on slow platforms which don't manage to achieve the full frame rate there is no evidence of this effect.

The obvious conclusion to draw is that it is a defect in Safari, but I'm reasonably sure it didn't happen when compiled with a much earlier version of Emscripten. It's a long shot I know, but does anybody have a clue what the cause might be?