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With alpha kept (Lock Transparent Pixels, Fill's Preserve Transparency), the brush, mixer, Fill, Gradient and Paint Bucket composited source-over and then restored the old alpha, so a half-transparent pixel took the new colour by as / ao instead of as. They now blend source-atop, as Photoshop does: Co = (1-as)*Cb + as*B(Cb,Cs), alpha unchanged. New shared helper blend::composite_atop; over_native's lock branch lerps by the source alpha. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>