These warnings were caused by a Vim process sending a DO message and then immediately exiting so that the process would some times die before the message was received by MacVim.
Most of the code shared between the text renderers now resides in a "helper" object. This way it will be a lot easier to maintain several renderers at once. As an added bonus, the ATSUI renderer now has mouse support as well as drag-n-drop support.
When opening a file from Finder, set the pwd to the directory the file resides in but only if the editor window is unused (e.g. it doesn't already have files open in it).