Direct2D is very strict about not using cleartype on a transparent surface. When on the URL in the bug, it seems when the top banner is loaded cleartype is gone, when you scroll it out of view, it comes back. This suggests we're somehow drawing to surfaces with transparency when the banner is in view.

The top part of the scrollable stuff is transparent and shows the fixed background underneath. So when it's in view, the scrolled surface is transparent. When it's not in view, the scrolled surface can be opaque.