yes it's true, there is such an optimization... Suggested by PeP... I always wanted to remove that experiment because it caused menu redraw problems... But then I forgot about it :) funny....
I was wrong once again, I had such an optimization in the windom version of netsurf, but that code has gone and now there are no optimziations of the rectangle list walking =)
However, JFL's hint is probably a good one and indeed it will have something to do with
topped windows.