Simple Dock: preview behind panel, sometimes

[LinuxOnTheDesktop]

 * Themes version/Build date (to be found in "About.." or in "metadata.json"): "last-edited": 1548100967
 * Cinnamon version (cinnamon --version): 4.0.9
 * Distribution - (Mint 17.2, Arch, Fedora 25, etc...): Mint 19.1
 * Graphics hardware *and* driver used: Intel onboard Mesa 3
 * Snippet or attachment of ~/.cinnamon/glass.log
[glass.log](https://github.com/linuxmint/cinnamon-spices-themes/files/2848328/glass.log)

 * Snippet or attachment of ~/.xsession-errors

Notify author of theme
@brownsr

Issue

Intermittently - even intermittently, I think, with one and the same application - thumbnails appear partially behind the panel.

Steps to reproduce

Possibly merely: have Firefox pinned to the panel; run Firefox; hover over the Firefox icon on the panel.

Expected behaviour

No overlap of thumbnail with panel. One does not get the overlap with other themes such as Mint-Y Dark.

Other information

I have HiDPI force enabled. My screensize: 2560x1440. I have top and side panels.