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In core_foundation/views/view/touch.js, the touchIsInBoundary method pulls in the touchBoundary property, does some very nice calculations with it, and then appears to ignore it at the last second:
if (x > 100 || y > 100) return NO;
Note to self (or anyone with an itch to get unit-testin'): fix this.
Also: refactor this to use SC.pointInRect, rather than rewriting that code here. (Thanks @publickeating.)
Fixed in master.