canvas's impl of this function is the same as nsFrame's:
*** Bug 373724 has been marked as a duplicate of this bug. ***
There's a small difference, http://mxr.mozilla.org/seamonkey/source/layout/generic/nsHTMLCanvasFrame.cpp#260 does an argument check. Is there a reason for this? Or can we just merge them?
You're looking at the seamonkey tree, I'm not sure exactly what that is but in mozilla-central nsHTMLCanvasFrame doesn't override GetContentForEvent. So this is fixed.