According to blink bug https://codereview.chromium.org/20049007, right now you webkit can eventually perform a O(n^2) scan through the document when moving the cursor up when at the beginning of a document, which is pretty bad not only when in design mode or in editable elements, but also when you have enabled the caret browsing mode (EFL & GTK ports).
Created attachment 208895 [details] Patch proposal plus new Layout test Attaching patch proposal, as backported from https://src.chromium.org/viewvc/blink?revision=154977&view=revision As in the case of blink (and as per Dominic's comment during the review process), the newly added test runs *very* fast (constant time) when the bug is fixed, and times out otherwise.
Thanks for the review, Darin. Unfortunately, I'm on holidays now and with no access to my devel machine until September the 3rd, meaning that I won't be able to commit this (or anything else) until then. Anyway, feel free to commit it yourself if you don't want to wait two weeks. Otherwise I'll do it right away once I'm back. Thanks!
Comment on attachment 208895 [details] Patch proposal plus new Layout test Clearing flags on attachment: 208895 Committed r154274: <http://trac.webkit.org/changeset/154274>
All reviewed patches have been landed. Closing bug.
*** Bug 119350 has been marked as a duplicate of this bug. ***