When is the gender of a shiny patch Pokemon determined?

Question

Is the gender of the Pokemon determined when you enter the patch or when the patch first appears on screen?

Comments

I would imagine it’s generated when the rest of the Pokémon is generated

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The Pokemon is generated when the patches spawn, so I believe the gender would be generated there as well

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I think the only way to test this would be to use an emulator and use save states. I don’t think there’s any information out there on this.

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Someone on Smogon claims it's determined when the grass starts shaking in gen 4, and they don't know about newer generations.

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I haven't done Poke radar in a while, but from what I remember from using cute charm, it seemed that gender was determined when the patch was generated, since I got the lower gender ratio with my Cute Charm Lopunny at the front of my party versus when I switched it to the front after the patch generated.