Another case of: Cannot read property 'left' of null

[jliebrand]

Latest fixes have definitely improved matters, but here's another set of coordinates that cause "Cannot read property 'left' of null":

https://gist.github.com/jliebrand/165f97f0812fe495ee013e00b715110d

[mfogel]

I've simplified the set of coordinates that triggers the bug. They're in the script issue-45.js on the bugfix/issue-45 branch.

Simplifying the coordinates further changes the error into a 'maximum call stack exceeded', like #44. As such, probably best to address #44 first and then return to this bug.