https://bugs.webkit.org/show_bug.cgi?id=135075 introduced a patch that would put fixed position elements into layers whenever the view is layer backed. This created a lot of problems for our test infrastructure, and until we can resolve some of those issues, it would be better if we just enabled the Setting on Yosemite since many more views become layer-backed without asking for it there. <rdar://problem/17682335>
Created attachment 235254 [details] Patch
Comment on attachment 235254 [details] Patch OK
Landed here - https://github.com/WebKit/WebKit/commit/d91046c9877fe0a663c0f29d5cbf95c4245575be