No particles appear in Downstream sim

[pixelzoom]

Originally reported in phetsims/qa#1100 by @KatieWoe. I moved this to its own issue.

---

I had one instance of a sim where none of the particles visually appeared. This was in state, on the bottom sim, on iPad Safari (latest). Refreshing the page seemed to fix it. I haven't been able to reproduce it since so I haven't made an issue yet. It may have been due to switching tabs or turning the screen off while the sim loaded, but I still haven't reproduced it. If a specific issue would help, let me know.

Edit: Was able to reproduce this, I think it is turning the screen off while the sim loads, but it doesn't happen every time.

[pixelzoom]

I discussed with @KatieWoe to clarify the steps to reproduce.

Before the State wrapper had fully loaded the sims, she switched browser tabs to do something else. When she returned to the State wrapper tab, she could interact with the Upstream sim, but no particles would appear in the Downstream sim. Other aspects of the Downstream sim (pressure, temperature, control values,...) looked correct. She was able to reproduce this a couple of time, but not on every attempt.

The direct link to the State wrapper for the dev test is https://phet-dev.colorado.edu/html/gas-properties/1.1.0-dev.18/phet-io/wrappers/state/.

And "iPad Safari (latest)" presumably means iPadOS 17.5.1.

[pixelzoom]

I have been unable to reproduce this on my iPad6 with iPadOS 17.5.1.

The only thing missing in the downstream sim is the particle system. The particle system is implemented using scenery Sprites, which uses WebGL. It's also the only thing in the sim that uses WebGL. So my guess is that switching away from the State wrapper before it was loaded was not allowing the WebGL context to be properly initialized.

Since the State wrapper is a demonstration of how state is saved/restored, and since switching away from it is not a normal use case, I will not investigate further and will close this issue. If it comes up again, we can revisit.