See the following reduction: http://ejohn.org/files/bugs/arc/ Which throws the following error: Error: INDEX_SIZE_ERR: DOM Exception 1 http://ejohn.org/files/bugs/arc/ (line 5) According to the canvas spec: http://www.whatwg.org/specs/web-apps/current-work/multipage/section-the-canvas.html "Negative values for radius must cause the implementation to raise an INDEX_SIZE_ERR exception." However, that is not the case for arcs with a radius of 0.
For posterity, here's the reduction: <canvas width="100" height="100" id="canvas"></canvas> <script>window.onload = function(){ var c = document.getElementsByTagName("canvas")[0]; var context = c.getContext("2d"); context.arc( 10, 10, 0, 0, 180, false ); };</script>
<rdar://problem/5808318>
Landed r31162