The new browser is using the GPU to do multi-layer compositing of the window contents instead of drawing using the CPU or an overlay.
Font rendering issues aside, it's an interesting problem to study because on a known good config, you have a localized window that is causing an issue.
I haven't tested it, I'm still on an older version, partly because I noticed newer versions had changed the font rendering slightly and I liked the way it looked better on the older one.