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In UI, TEXCOORD0.zw, TEXCOORD1.zw, TEXCOORD2.zw ... components will be discarded.
So you can use only TEXCOORD0.xy, TEXCOORD1.xy, TEXCOORD2.xy ... components for custom vertex stream.
TEXCOORD0.zw, TEXCOORD1.zw, TEXCOORD2.zw ... components must to be filled with "unnecessary" data (such as Size.xy.)

In addition, add the channels you want to use to Canvas.AdditionalShaderChannels.

经查询

2019版本及以前 UIVertex中 uv 是 Vector2 的。
2020版本及之后 UIVertex中 uv 是 Vector4 的。