Go into a bug review, like https://bugs.webkit.org/attachment.cgi?id=48732&action=review Place the caret in the middle of a line. Hit command-left arrow to move the caret to the left edge. Hit shift-down arrow a few times to select some lines Hit down arrow once to deselect and move to the next line. At this point you'll see a bogus selection rectangle left to the right of the previously selected lines. This didn't use to happen.
I think the steps to reproduce are missing one crucial step: before pressing the down arrow key to clear the selection, scroll the text area vertically.
Regression->P1
<rdar://problem/7650652>
Created attachment 49034 [details] Patch with repaint test
Fixed in <http://trac.webkit.org/changeset/54982>.