Hovertool display pops under shaded portion of slider #7991
Description
ALL software version info (bokeh, python, notebook, OS, browser, any other relevant packages)
bokeh 0.12.16
python 3.6
MacOS 10.12.6
Chrome Version 66.0.3359.181 (Official Build) (64-bit)
Description of expected behavior and the observed behavior
I'd expect the hovertool info box to appear above all other widgets. Instead it pops under the slider, but only the shaded portion of the slider as demonstrated below.
Screenshots or screencasts of the bug in action
