The repaint test that I'm about to commit for bug 23093 shows that the cancel button's renderer is 13x11px, which is too short. This causes incomplete redraws. It also makes the bottom 2px of the cancel button onclickable.
The cancel button size is hard-coded in RenderThemeMac to be 13x11px at normal font sizes. It draws correctly because when drawing, we call [NSSearchFieldCell cancelButtonRectForBounds:], which returns a rect that is 19x19px.
Note that just changing the cancelButtonSizes constants causes the search field to layout incorrectly.
Created attachment 28463 [details]
Repaint test result showing short rect
When fixing this, should also fix the cancel button with zooming.