User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:2.0) Gecko/20100101 Firefox/4.0 Build Identifier: http://download.mozilla.org/?product=firefox-4.0rc2&os=win&lang=de Compositing with globalCompositeOperation="copy" seems to be flawed in FF4.0RC2. Please see the attached screenshots and code for details. The behaviour is correct in FF3.6 and Chrome 11. InternetExplorer 9.0 and Opera 11 behave the same as FF4.0. Note: The shown case might be a corner case not explicitly formulated in the HTML5 Specification thus it might be controversial if this is a bug. Also so far i have not found any official test cases checking for this case. I believe however the behaviour in FF3.6 and Chrome 11 is correct. Reproducible: Always Actual Results: see attachments
For that testcase, the behavior change range is: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=3202d6d22192&tochange=abb82f981e02 So presumably this is a result of bug 366283. If I read that bug correctly, this is a purposeful change to match what the canvas spec says.
Yes, FF4, IE9 and Opera 11 are following the spec, and Chrome and FF3.6 are not.