It´s great to finally have folding in a native osx application. Although the current implementation has a few usability issues:
First, the folding column needs to be rightmost in the gutter, next to the text. Currently, when line numbering is active, matching folding widget with line is harder than it could be.
Second, the present top and bottom widget icons are too similar, making it hard to read a structure overview off the document. It´s primary function is to show start or end of the block, thus arrow direction must be the most visible feature.
In addition i suggest a highlighting feature, allowing us to close a fold(?) by clicking anywhere on a line between between folding icons. A mouse over effect shows what parts will be affected.
This will allow easier matching of open and closing arrows, as well as reducing mouse travel.
View the attached mock-up for a clearer, visual description. (note in the image I am experimenting with a x-code style top-bar as well ;)
hp.