I'm not sure this is completely correct. We pass the offset to pushShaderMask as well, which is redundant with the rect that we pass now. I'd look at how the other functions (e.g., clip) handle this case. I thought we always passed the rects before applying the offset.

I guess the tricky issue here is the shader callback?

The offset is passed to pushShaderMask so it can be passed onward to the painter() callback.

pushClipRect() internally applies the offset to the clipRect and uses the result to construct the ClipRectLayer.

We could do something similar here - call _shaderCallback(offset & size) in RenderShaderMask.paint to build the shader. Then, in pushShaderMask, use maskRect.shift(offset) as the maskRect when constructing the ShaderMaskLayer.

Would that be more consistent?

Yes, I think so.