Playing with CSS transparency and blur filter to simulate "frosted film" translucency effect featured in OS X Yosemite...
Current browser support is limited, however: http://caniuse.com/#feat=css-filters
Update 2018.04.16: Blurred :before element's size extended beyond visible bounds to eliminate blur falloff of the filter. Added $blur variable for easier manipulation.
Update 2018.05.30: Replaced jQueryUI with D3 for dragging.