After 18 months they've updated Prototype with some significant new features.
Downgrade bundled Prototype version back to 1.7
Integrated in tapestry-trunk-freestyle #977 (See https://builds.apache.org/job/tapestry-trunk-freestyle/977/)
TAP5-1989: Upgrade bundled Prototype to version 1.7.1 (Revision 7b397e31d3a7c1c13bd3b23d11813df5a104015a)
Result = FAILURE
I suspect the upgrade in 5.3.5 of prototype to 1.7.1 is the source of the problem.