Concave corner offsets

#532190484AssignedP3 / S3Blink > Paint

Simplified reproducer

Live rendering

An asymmetric concave top-left corner is combined with an outline offset and a zero-blur box-shadow spread. Before the fix, the calculated miter escaped the target edge and produced a long stray segment.

Expected

Fixed output captured from the locally built Chromium content_shell at device scale factor 1.

Expected fixed content_shell rendering

Minimal CSS

div {
  width: 80px;
  height: 80px;
  background: red;
  border-top-left-radius: 10px 50px;
  corner-top-left-shape: superellipse(-0.5);
  outline: 5px solid currentColor;
  outline-offset: 20px;
  box-shadow: 0 0 0 15px green;
}

Fix

PathBuilder::AddContouredRect() now follows the CSS Borders 4 corner clip-out algorithm, using radius-aware normals and tangent miters while clipping self-intersecting concave outsets. The same geometry path covers outline offsets and shadow spread.