STR: 1. Set layers.acceleration.disable = true and layers.offmainthreadcomposition.force-basic = true 2. Go to the extensions list on about:addons. 3. Scroll the list. Stuff isn't erased properly, paints beyond the scroll box's bounds and generally looks broken.

The existing code ignored aMaskTransform completely. The NS_ASSERTION probably meant to check aMaskTransform but instead checked "matrix", which is the transform from sourceRect to aDestRect. Scrolling changes the transform that the DrawTarget comes with. aMaskTransform seems to be relative to the window, so it looks like it should ignore any existing transform on the DrawTarget. That's why I replace the transform on the DrawTarget with aMaskTransform. The old transform is still used in the pattern transform.