when you setup a sprite with +spritedir-sprite(icon-large, $offset-x: 100%, %offset-y: 50%) - the resulting sprite gets a background position of: 100% 50px.
Any number given to $offset-y is converted to px. This is all fine and dandy for vertical sprites, but horizontal ones need to do the 100% to the y offset.
Line 158 in compass / lib / compass / sass_extensions / functions / sprites.rb checks the x offset for percentage and uses it if there is, but nothing is done for the y offset - it just parses out the number and puts the px on there.
Was pretty simple... I couldn't find tests for this area, so I didn't include any.
is not finished.
Should be on top of the changes coming in #1771