I've faced weird issue using canvas clearRect in Firefox. If clipping region is set, clearRect() redraws a content which is outside both clipping region and rectangle to clear. Since it redraws it over this causes an issue. It works correct in other browsers, like Chrome and IE - clearRect doesn't affect the content outside clipping region.
Here's the code:
var canvas = document.getElementById("test");
var context = canvas.getContext('2d');
context.fillStyle = 'rgba(255, 0, 0, 0.1)';
context.fillRect(0, 0, 40, 40);
context.beginPath();
context.rect(0, 0, 20, 20);
context.clip();
context.clearRect(10, 10, 20, 20); // causes an issue, see example below
Code in action: http://jsfiddle.net/P8fyP/4/
I tested your code at firefox 18 and firefox nightly 21 and I can't find a problem. The code do what it is supposed to do.
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