Bug 57647 represented transparent windowless plugins being incorrectly composited with the page's content. I had trouble writing a test case but wanted to get the fix landed. This bug represents writing a test case.
Maybe my test case difficulties were caused by using GDI, which is not really alpha-aware.
Bug 60300 solves part of the mystery of why I couldn't get transparent plugins to show up.
Specifically, I was using ::GradientFill in my plugin, which zeroes out the alpha channel of the bitmap. Presumably I could have gotten the plugin to show up had I manually filled in the alpha channel with 1, or used ::AlphaBlend with a constant opacity.