This seems to be a problem with the differences in scroll step or the size of the items in the list. The thing is that 10 attempts with an scroll amount of 10 is not enoug in the GTk+ port to scroll the list to the desired point. Either increasing the scroll amount or the maximum attempts would fix the test. Increasing the scroll amount would break the test in other ports so better use a higher number of maximum attempts.
Created attachment 300893 [details] Patch
Comment on attachment 300893 [details] Patch We expect our list items to use theme size and not match other ports, I presume?
Committed r211872: <http://trac.webkit.org/changeset/211872>