This is easy to reproduce by using the MiniBrowser in editable mode, try something like this:
MiniBrowser --bg-color="rgba(255,0,0,1)" -e
Write someting and move the cursor or select text, you will see that some areas are not repainted at all. This is because when using a web view color, we fill with the color every rectangle updated by the web process, but we should always fill the entire backing store before rendering the actual contents on top.
Created attachment 273422 [details]
Comment on attachment 273422 [details]
This doesn't work, sometimes we render the bg color over the contents
Created attachment 273423 [details]
This fixes the problem from the web process side. It's a it more aggressive, because it always sends updates to the Ui process for the whole visible content rectangle, but it's the only way I've managed to fix the problem.
Comment on attachment 273423 [details]
I've been poking at this code recently and this change makes sense to me. r+
Committed r197927: <http://trac.webkit.org/changeset/197927>