Created attachment 333860 [details] testcase The two attachments should look the same since they have pretty much the same DOM. "Testcase" appears blank while "reference" shows a broken-image icon. I think they should both display broken-image icons. My "refdyn" tool found this bug based on the reftest for bug 395390. Bug 408782 might have been similar.
ShouldCreateImageFrameFor looks at whether the alt attribute is present, but there is nothing hooked up to make a dynamic change to it trigger a frame reconstruct... There are probably other ways that ShouldCreateImageFrameFor can cause hysteresis issues like this.
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