See the upcoming testcase. When a canvas element has a border and has its opacity changed from 1 to <1 its height shrinks by the border height and does not return to normal when the opacity returns to 1. If the opacity begins as <1 (even as 0.9999) the initial height is the same as the opacity 1 case but the height never drops.
Created attachment 203643 [details] Testcase Here is the testcase. The box is a canvas element. Moving the mouse over it will change it's opacity from 1.0 to 0.5.
Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8) Gecko/20051119 Firefox/1.5 ID:2005111903 confirmed
Hrm, I don't think this is canvas itself; maybe roc has some input on what it could be?
This looks like it's worksforme